Fuel economy of flat six

[55 Fargo]

2 minutes ago, Robert Horne said:

My new 1980 Chevy Van, with the 6, came with the 2 barrel, that ran on one barrel, then the second barrel kicked in later.

The Van did ok, if on level roads, and no extra weight, and got 10 mile to the gallon.

I pulled the engine, and installed a 400 small block, and got up to 17 mile to the gallon...

I had a swb 1982 Dodge van, 225 slant 6, 25 mpg easy on highway all day long. With a wind on my tail upwards to 30 mpg.

Buy Dodge......LOL

[Dartgame]

If fuel economy and power are the goal, then both can be achieved with a progressive 2 bbl like sser2 describes. An overdrive is probably the other big change that would help along with a highway rear axle gear, or at least something under 3.50.

If I had a 2 bbl intake, I'd experiment with it to see how a progressive 2 bbl works. But too much other stuff to address on my car, meanwhile the engine runs terrific as it sits, so why mess with it. someday I intend to explore this kind of stuff.

It would be great to hear if someone has done the 2 bbl conversion.

[Robert Horne]

In the stock setup on my 38 Coupe, 80 hp 218/3 speed trans, stock rear (around a 4.00 gear), I was getting around 15 MPG.

After I installed the 5 speed trans, 3.73 rear, I was getting MPG in the 20s around town, and to the cruise ins...

On the club rod run of 100 miles, I used around 3 gallons of fuel.    The speeds were mostly 40 to 55 MPH.

My 300 Chrysler with the 3.6/250Hp engine/8 speed trans, gets about 22 MPG around town, and 32 MPG on a trip..

I would guess at 50MPH, my 38 has as good a wind design as my boxey 300...............

 

[knuckleharley]

47 minutes ago, Dartgame said:

If fuel economy and power are the goal, then both can be achieved with a progressive 2 bbl like sser2 describes. An overdrive is probably the other big change that would help along with a highway rear axle gear, or at least something under 3.50.

If I had a 2 bbl intake, I'd experiment with it to see how a progressive 2 bbl works. But too much other stuff to address on my car, meanwhile the engine runs terrific as it sits, so why mess with it. someday I intend to explore this kind of stuff.

It would be great to hear if someone has done the 2 bbl conversion.

I'd like to hear if someone has done this with the old Edmunds 2x2 intake for the 23 inch Mopars. I have one and have been curious for a while about how people could run that intake and not bog the engine down with gas

[55 Fargo]

5 minutes ago, knuckleharley said:

I'd like to hear if someone has done this with the old Edmunds 2x2 intake for the 23 inch Mopars. I have one and have been curious for a while about how people could run that intake and not bog the engine down with gas

Well you just might, with a stock engine knuck.

But a 230 high compression head, hi-perf cam, racing pistons, might be another movie.

i thought my 228 would be bogged, with dual truck carbs the biggest available in Carter B&B, no issues whatsoever.

The fuel from 2 or 3 carbs on this engine design just flows and delivers way more efficiently.

Otherwise they would or could just make single intakes for small 4bbl carbs, not efficient at all with this design.

 Knuck as you already know, think huge vacuum pump, the more pressure the more intake and exhaust....

[Jeff Balazs]

12 minutes ago, knuckleharley said:

I'd like to hear if someone has done this with the old Edmunds 2x2 intake for the 23 inch Mopars. I have one and have been curious for a while about how people could run that intake and not bog the engine down with gas

Yes me too. And if anyone is running a pair of webers on something like this.

[knuckleharley]

6 minutes ago, Rockwood said:

Well you just might, with a stock engine knuck.

But a 230 high compression head, hi-perf cam, racing pistons, might be another movie.

i thought my 228 would be bogged, with dual truck carbs the biggest available in Carter B&B, no issues whatsoever.

The fuel from 2 or 3 carbs on this engine design just flows and delivers way more efficiently.

Otherwise they would or could just make single intakes for small 4bbl carbs, not efficient at all with this design.

 Knuck as you already know, think huge vacuum pump, the more pressure the more intake and exhaust....

Well,the truth is that back when these 2x1 and 2x2 intakes were being made there WERE no such things as 4 brl carbs. I have a 54 Olds with a 324 and 4 brl carb from the factory,and that was the first year Olds put a 4 brl carb on any engine. I also have a 55 DeSoto 291 S engine that came from the factory with a 4 brl. 55 was the first year for Mopar 4bls,and they were optional.

It was the introduction of OHV V-8 engines that put both the flat 6 and 8 engines into scrap piles,as well as the old custom multiple 1 barrel carbs people used to hop them up.

55 was also the first year for a 4brl on FoMoCo stuff. My father bought a new 55 Victoria with a 272 and 4brl carb.

[50plymouth]

On January-10-17 at 3:08 PM, knuckleharley said:

I can tell you how it relates,fuel economy for Mopars is determined no differently  than it is for any other car. Some things are universal. Driving habits is one. Flathead engines is another. The 51 Ford I sometime write about here is powered by a flathead 6 cylinder engine. Do you think factors involved with fuel consumption in a flathead Ford 6 cylinder engine are different than those with a flathead 6 Mopar engine? Do you think flathead 6 Ford powered cars have to be driven,maintained,or modified differently to obtain the same fuel mileage or power increases? Certain things are basic and apply to all engines.

Its clear you will not get a response from the person you have quoted, but have you considered that it is that type of response that leads to such people leaving ?  She could likely respond back that yes there are different factors for fuel consumption between a flathead 6 cylinder ford and a 6 cylinder mopar as there are differences.  It does not appear to be that which drew the response in my mind as much as yet another thread that started off talking about p15-d24 mopars and which quickly got dominated with other manufacturers. Those posts I don't read, nor I expect anyone would, as how to improve a flathead mopar.  When the thread gets to the point where posters feel their 1997 Ford 150 is relevant, its easy to imagine why people start to leave.  For me really the question is why should I stay?   I know Knuckleharley your heart is in the right place as much as I know there can be things from one manufacture that can improve another.  I would just ask before you either respond to this note or drop me a PM if you wish to discuss further, that you read the original post on the thread. Then read all the posts on the thread and make a judgement to whether you truly believe this thread like too many goes off the trail and into the bush.

To the topic at hand I have long ago achieved the 30 mpg goal.    It was a challenge among friends when we realized we likely were not going to be able to compete in the fastest car category. It was likely more so what our bring home pay was and as cheap as fuel might now seem, it was still costly at the time.

The winning formula is actually fairly simple. A light weight 1936 Plymouth with a 308 gear set placed in it. an r6 overdrive from a Desoto. Dual carbs and dual exhaust and a device from a 1935 Chrysler that allows you to adjust the distributor while driving. It is a knob that through a cable connected to a plate under the distributor allowed you to advance or retard the timing.  From Buffalo to Rochester the car got 32 mpg.  It was a cool damp may drive and the car was running what was the high octane fuel of the day.

The issue with the car was while it was great to drive on the highway when you got up to speed, it took forever to get to speed. At that point in time we were thinking higher top speed at lower rev and saving fuel.    I am positive I can point you to several guys way smarter than I who could do a better job to get you to 30 mpg today. From the folks I have hung around most are most interested in getting a little more power and better highway driveability than they are achieving 30 mpg.

 

[50plymouth]

3 hours ago, knuckleharley said:

I'd like to hear if someone has done this with the old Edmunds 2x2 intake for the 23 inch Mopars. I have one and have been curious for a while about how people could run that intake and not bog the engine down with gas

I have and I have had friends with them. I believe Edmunds made them to address part of the market that thought 2 barrels would be better than the single barrel carbs of the day. In my case I got one because when Edmunds came out with a much better design, the hot rodders who had the 2x2 took them off and I got one for $20. The manifold was too long and it was not a great design. It didn't bog but the engine lost its torque at the bottom end. It seemed to rev higher. I tried an adapter they had that allowed you to put a single carter ball and ball on the intake. It certainly ran better.    Looking back I am not sure if it wasn't the fact that the adapter put the carb further up in the air.  My best friend put webers on his with similar results. He made his own adapter plates to raise the webers up off the intake and it helped.  I sold that car, while he finally save the $65 and bought the later Edmunds intake and linkage. He put two Dodge truck carbs from a late 40s Dodge truck on that intake and 60 years later, still has the car with that combination.

[Don Coatney]

On 1/8/2017 at 8:22 PM, sser2 said:

11. Good driving habits. Maintain steady speed, minimize hard braking and acceleration, use pulse-glide where appropriate. There are devices that show instant mileage - they are very helpful in learning how to drive for economy. Unfortunately, they only work with fuel injection. A manifold vacuum gauge on dashboard is acceptable substitute. Economy driving is safe driving.

Critiques? Other suggestions?

Item number 11 from the original poster.

It has been my experience having driven many years with many different vehicles that "good driving habits" has as much if not more to do with better gas mileage than many mechanical alterations no matter what manufactures equipment is used. Therefore the information about driving habits I posted about my 1997 F-150 is relevant to this thread. 

My wife has heavy feet on both the brake and accelerator pedals. Her car has an instant readout on miles per gallon. When I drive her car I can increase the MPG readout by 20 to 30%. More proof that good driving habits can have a big effect on economy.

[T120]

I have really enjoyed reading this thread through. There's been an excellent discussion on modifications tried and results obtained as well driving habits being mentioned. Sometimes things get a little off subject and I was sorry to read that someone was offended by this and they announced, "At the risk of starting a bush fire..." - It didn't. As a reader, I simply discard that which doesn't interest me. Actually, the only thing I saw that may be considered inflammatory was that some was told to .."get a life ..." . It was acknowledged by the person addressed and shrugged off...

[knuckleharley]

4 hours ago, 50plymouth said:

Its clear you will not get a response from the person you have quoted, but have you considered that it is that type of response that leads to such people leaving ?  She could likely respond back that yes there are different factors for fuel consumption between a flathead 6 cylinder ford and a 6 cylinder mopar as there are differences.  It does not appear to be that which drew the response in my mind as much as yet another thread that started off talking about p15-d24 mopars and which quickly got dominated with other manufacturers. Those posts I don't read, nor I expect anyone would, as how to improve a flathead mopar.  When the thread gets to the point where posters feel their 1997 Ford 150 is relevant, its easy to imagine why people start to leave.  For me really the question is why should I stay?   I know Knuckleharley your heart is in the right place as much as I know there can be things from one manufacture that can improve another.  I would just ask before you either respond to this note or drop me a PM if you wish to discuss further, that you read the original post on the thread. Then read all the posts on the thread and make a judgement to whether you truly believe this thread like too many goes off the trail and into the bush.

To the topic at hand I have long ago achieved the 30 mpg goal.    It was a challenge among friends when we realized we likely were not going to be able to compete in the fastest car category. It was likely more so what our bring home pay was and as cheap as fuel might now seem, it was still costly at the time.

The winning formula is actually fairly simple. A light weight 1936 Plymouth with a 308 gear set placed in it. an r6 overdrive from a Desoto. Dual carbs and dual exhaust and a device from a 1935 Chrysler that allows you to adjust the distributor while driving. It is a knob that through a cable connected to a plate under the distributor allowed you to advance or retard the timing.  From Buffalo to Rochester the car got 32 mpg.  It was a cool damp may drive and the car was running what was the high octane fuel of the day.

The issue with the car was while it was great to drive on the highway when you got up to speed, it took forever to get to speed. At that point in time we were thinking higher top speed at lower rev and saving fuel.    I am positive I can point you to several guys way smarter than I who could do a better job to get you to 30 mpg today. From the folks I have hung around most are most interested in getting a little more power and better highway driveability than they are achieving 30 mpg.

 

No. Why should I? She obviously loves rules more than knowledge,and nothing less than allowing her to monitor and censor each post will please someone like her.

[knuckleharley]

4 hours ago, 50plymouth said:

I have and I have had friends with them. I believe Edmunds made them to address part of the market that thought 2 barrels would be better than the single barrel carbs of the day. In my case I got one because when Edmunds came out with a much better design, the hot rodders who had the 2x2 took them off and I got one for $20. The manifold was too long and it was not a great design. It didn't bog but the engine lost its torque at the bottom end. It seemed to rev higher. I tried an adapter they had that allowed you to put a single carter ball and ball on the intake. It certainly ran better.    Looking back I am not sure if it wasn't the fact that the adapter put the carb further up in the air.  My best friend put webers on his with similar results. He made his own adapter plates to raise the webers up off the intake and it helped.  I sold that car, while he finally save the $65 and bought the later Edmunds intake and linkage. He put two Dodge truck carbs from a late 40s Dodge truck on that intake and 60 years later, still has the car with that combination.

Well,I paid a little more than 20 bucks for the one I have,but I was guessing I would hear pretty much what you wrote. Probably work awesome with a maxed out race engine,

[greg g]

Growing up we had a interesting mix of vehicles in the neighbor hood.  Dad had a 54 Meadow Brook with a 6 with fluid drive 3 speed. The next door neighbor had a 53 Studebaker coupe with 6 3 speed.  The folks across the street had 55 Ford wagon with a 292 V8 and fordomatic two speed. There was a 6 cylinder Henry J and a pre war 6 cylinder Pontiac. My uncle had a 53 Buick straight with Dynaflow.  I recall conversations about gas milage.  My Uncle never had much to say, the Studebaker and the Henry J were always mentioned in the high teens, dad seemed happy with any thing over 15 with the Dodge. The Ford seemed hard pressed to achieve anything over 10mpg, and the Pontiac came in around 12. The Studebaker got traded for a Renault Dauphine which got more than 25 mpg. Dad traded the 54 for a 60 Dodge Dart with a /6 standard that gave him about 20 in general use (he was always a very smooth driver who guarded momentum, and watched traffic flow to minimize throttle changes and unnecessary braking).  He was later amazed when a 65 Coronet 318 three speed improved his economy to 23 mpg. The my friends brother bought a mini and talked about 35+ mpg on his trips from the Brooklyn Navy Yard to home in Central NY.  Proper gearing, good technique, proper tire inflation, and good tune up all come together she you want to chase peak mpg.

[knuckleharley]

On 1/13/2017 at 6:55 PM, greg g said:

Growing up we had a interesting mix of vehicles in the neighbor hood.  Dad had a 54 Meadow Brook with a 6 with fluid drive 3 speed. The next door neighbor had a 53 Studebaker coupe with 6 3 speed.  The folks across the street had 55 Ford wagon with a 292 V8 and fordomatic two speed. There was a 6 cylinder Henry J and a pre war 6 cylinder Pontiac. My uncle had a 53 Buick straight with Dynaflow.  I recall conversations about gas milage.  My Uncle never had much to say, the Studebaker and the Henry J were always mentioned in the high teens, dad seemed happy with any thing over 15 with the Dodge. The Ford seemed hard pressed to achieve anything over 10mpg, and the Pontiac came in around 12. The Studebaker got traded for a Renault Dauphine which got more than 25 mpg. Dad traded the 54 for a 60 Dodge Dart with a /6 standard that gave him about 20 in general use (he was always a very smooth driver who guarded momentum, and watched traffic flow to minimize throttle changes and unnecessary braking).  He was later amazed when a 65 Coronet 318 three speed improved his economy to 23 mpg. The my friends brother bought a mini and talked about 35+ mpg on his trips from the Brooklyn Navy Yard to home in Central NY.  Proper gearing, good technique, proper tire inflation, and good tune up all come together she you want to chase peak mpg.

Greg G,55 Fords came with the 272 V-8,and the Fordomatic was a 3 speed transmission until Ford started putting it in Falcons,etc,in 1959.  Actually,it continued well into the 60's as a FMX and a Cruise-o-matic. My 1951 Ford Fordomatic is a 3-speed. The difference is the ones with the name Ford-O-Matic normally started off in second gear. If you wanted to start in 1st gear,you had to shift down and then shift up for 2nd and high gear. I am GUESSING Ford did this to make takeoff's smoother.  BTW,the early Studebakers used the same transmission up until some time in the mid to late 50's.

IIRC,it was when the FMX came out that the Fordomatic started out in first gear and shifted automatically to 2nd,and then 3rd gear. You can put a FMX valve body in a early FOM and it will start out in 1st gear,too. The early FOM's are easily identified because they have the cast iron cases. 1951 and 1952 FOM's had a shorter tailshaft than the 53's and later,but to check or add ATF in either you had to pull up the carpet,remove the inspection plate on the trans tunnel,and then pull out the dipstick. It was about the size of the dipsticks you see in PS pumps,for reference. In 52 Ford went with the tube under the hood to check and add ATF.

The 59 and later 2-speed FOM's had cast aluminum cases.

 

BTW,my father bought a brand new Victoria 2dr ht in 55 with the 272 with the 4brl option,and FOM. PS,but no pb or ac. The best gas mileage he ever got was 14 MPG. Frankly,the early intakes and teapot carbs sucked. I had a full size custom 300 in 1968 with the 292 and 2brl carb,and I got up to 22 mpg coming home from Ft.Bragg on weekends running 65-70 mph when I could,and taking no care whatsoever to try to get better gas mileage. Not sure about  the rear gearing,but am assuming in was 3:25 or higher because the car didn't have OD.

Our next door neighbor had a 1st year  (1959?) standard shift Plymouth Valiant with the 198 cube slant 6 and a floor shift standard trans (3 or 4 speed? OD? Anyone know?),and she got around 20 mpg driving around the city,which at that time had never heard of a traffic jam. Since she went from a hemi DeSoto 4dr to the Valiant,she was VERY aware of the increased gas mileage.

IIRC,I even got a little better than 18 MPH on the road with my 71 340 Duster,and that thing would FLY. Once again,taking no care whatsoever to get better gas mileage.

Edited March 21, 2017 by knuckleharley
correct errors

[greg g]

The Ford folks probably never engaged low on that trans so they essentially operated it as a two speed.  The guy with the Pontiac never used second gear.  He would back out of his driveway, go about 15 feet in first then shift directly to high, using the torque of the big six to leisurely pull him up to 25 mph, which seemed to be his never exceed speed.  In 63 or so his daughter took the car after he become I'll, I think she said the car had 6000 miles on it after 22 years of I ownership.

[knuckleharley]

1 hour ago, greg g said:

The Ford folks probably never engaged low on that trans so they essentially operated it as a two speed.  The guy with the Pontiac never used second gear.  He would back out of his driveway, go about 15 feet in first then shift directly to high, using the torque of the big six to leisurely pull him up to 25 mph, which seemed to be his never exceed speed.  In 63 or so his daughter took the car after he become I'll, I think she said the car had 6000 miles on it after 22 years of I ownership.

When you stop and think about it for a minute,the Mopar fluid drive,and the FOM were probably the two most useful advances in  auto design for people that lived in snowy areas of the country. Especially hilly and snowy areas. You could/would take off in 2nd gear with both,and there was less chance of tire spin when the roads were slick. Really important when you are stopped going up a hill by a stop sign or red light and would normally have to take your foot off the brake to use the accelerator while letting out on the clutch. The Fluid Drive and the FOM took most of the drama out of that.

My 42 Dodge business coupe was bought new by a woman in Boston on the 3rd of December,1941. She traded in a 41 Dodge business coupe on it and kept and drove the car until she died in 1973,so she must have really liked the 42's.  I know this because I got the original bill of sale from the Dodge dealer in Boston with the car,and the notice of her estate sale listing the Dodge. It was parked inside it's whole life,and apparently she didn't drive it very much because it only has right at 30,000 on the odometer. The guy I bought it from bought it from her estate sale,and left it parked outside in his back yard in NH. He was restoring a business coupe,and bought it so he could look at it when putting his back together to make sure it went together right. I didn't buy it until a couple of years ago,so it sat outside with grass all around it in the summer,and snow all around it in the winter. By the time I got it even the front crossmember is rusted away. Mouse/rats nest in it. The car is so rusty that the guy I sent to get it called me on the phone and asked me if I really wanted him to bring it to me. Since I had already paid for it and had to pay him for going to get it anyhow,I told him to bring it to me.

On top of that the trunk full of garnish and other parts that were supposed to have gone with the car were not with it when it showed up,and neither were the distributor,carburetor,or the hubcaps. All of which were on the car in the photos.  When I called the seller on the phone to ask him about the missing parts that I was told went with the car,he told me "My son came by to pump up the tires and help me move it around to the front for your guy to pick it up,and he said something about knowing a guy looking for some of that stuff,so I guess he took it and sold it." Just like that was no big deal.

The seller also lied to me about the rust. I called him to ask about rust and he told me the car wasn't rusted out anywhere,and what I was seeing was just surface rust. There were holes in the floor big enough to throw a cat through when the car was delivered,and the whole trunk floor was missing . That,and the rotted out bottom of the front crossmember were why the car transport guy called me on the phone. I now have almost all the missing parts replaced,but still need a replacement front cross member.

 

I either go after anything I buy now and bring it home myself,or I don't buy it.

[JerseyHarold]

I'm a little late to this party but remember that my father's original '52 Plymouth consistently did 17 mpg in the city (Brooklyn...lots of stop and go) and 23 mpg on the highway.  He was a NYC cab driver for many years and was adept at making good shifts.   He also maintained the tire pressure at 32 PSI year round, which may have helped. 

[50plymouth]

22 hours ago, knuckleharley said:

Well,I paid a little more than 20 bucks for the one I have,but I was guessing I would hear pretty much what you wrote. Probably work awesome with a maxed out race engine,

No their a failed flawed design.  No one with a maxed out race engine ran that intake and Edmunds dropped the design quickly. All of his later model designs were higher up in the air to regain torque and dual barrel concept dropped. Sorry, as perhaps you were hoping this to be some sort of rare performance piece. It is not. You can get better throttle response and more cfm from the carter ball and ball configurations.

[knuckleharley]

9 minutes ago, 50plymouth said:

No their a failed flawed design.  No one with a maxed out race engine ran that intake and Edmunds dropped the design quickly. All of his later model designs were higher up in the air to regain torque and dual barrel concept dropped. Sorry, as perhaps you were hoping this to be some sort of rare performance piece. It is not. You can get better throttle response and more cfm from the carter ball and ball configurations.

Well,basically I bought it because I had just sold something else and had some "free range money" in my pocket,it was all show-polished and purty,and I wanted it. I also have a couple of 23 inch Mopar blocks it will bolt onto.

Plus at that time I didn't have any multi-carb intakes for my 23 inch Mopar engines. I pretty much knew it was more for looks that actual performance in a daily driver,but at this stage in my life I am more interested in re-creating 50's style hot rods than I am maximum performance,be it top in horsepower or gas mileage. It's just something that would make me smile and make me happy everytime I raised the hood.

I was kinda/sorta thinking I might run it temporarily,but when I got bored I would start doing stuff like making spacers to raise the carbs,and even making raised adaptors to bolt 1 brl carbs to it to see what would happen. I now have other intakes for the 23 inch engines,and may just end up putting it up for sale on ebay or craigs list or try to trade it for something. Don't really care one way or the other. It's paid for and it doesn't eat anything,so it can just sit on the shelf so I can smile at it will walking past and that's ok with me. I never allow myself to get stressed over spent money. I can do all the stressing I need to do over money I have to spend next month to worry about what I spent last month.

The truth is none of this stuff belongs to me,anyhow. I'm just babysitting it until someone else buys it  to babysit at my estate sale

[Niel Hoback]

I like your attitude.

[55 Fargo]

Interesting thread, not sure if its a "trans" thread a Hi Perf thread or a maximum fuel economy thread, but it is sure all over the map with some right off topic vehicles included.

Interesting exchange of information, some ground in facts, some pure conjecture and some wishful thinking.

But it's almost as entertaining as a "best oil" thread.....

 

Edited January 14, 2017 by Rockwood

[knuckleharley]

11 minutes ago, Niel Hoback said:

I like your attitude.

Thanks.

[Flatie46]

I like the the idea of getting more fuel efficiency from any old tech engine. I've always liked to try to get the most out of any engine. Often when tuning and improving performance you will gain fuel economy and vise versa. I love to read where people try new things on these old engines. I can't remember if I read it here or on the HAMB about the guy who put a turbo on a flathead 6. (their's a youtube video on this) http://www.bing.com/videos/search?q=kevs+turbo+flathead+onyoutube&view=detail&mid=47F6932B46FDFBE9CA8C47F6932B46FDFBE9CA8C&FORM=VIRE  The flathead engine design is pretty inefficient, that's why it was dropped so many years ago. The engineers pretty much stopped contributing to it's design well before it even ended production other than a few minor changes. I'm sure if you applied some modern technology there's no telling what you might come up with. I've been thinking of building  my own alum intake and making my own headers. I've got access to a mill at work and I can tig weld, never know. I need to apply what little time I have to getting my old car moving stock first I think though.  I understand guys saying 16 mpg is good enough, didn't buy it for MPG's. I also like seeing what kind of MPG's can be milked from one. I do like drinking a cold beer, reading these posts, with the AFC play offs going on in the background. LOL. Good luck with it, keep us posted on it. 

[55 Fargo]

A picture is worth a thousand words, don't chase your tail over gas mileage