Best Differential Gear Ratios and Why?

[55 Fargo]

Okay been a lot of trans questions and differential gearing questions, engine RPM reline limits etc lately.

A lot of the newer members, usually younger and most never experienced a low geared truck, let alone a engine driven rad fan.

So tell us, what is your Diff gear choices and why, this can include transmissions, whether stock, or some more modern Overdrive types.

Geographic locations, local traffic and speed considerations, as well as terrain types.

I currently am running 3.23 rear gears and a T 98 acme 4spd trans, this with a 228 long block, dual carbs/exhaust.

This current ratio, is a little tall in 4th gear, at 40 mph, which around 1800 rpm, she is a bit sluggish, once I hit 50 mph she spins up a lot better, doing 70 mph, she is not running all that hard at about 2800 rpm.

I see an optimal cruise RPM at 2500-2800 rpm, yes 3000 should be no problem, but 3400-3600 on a continuous, gets busy and is running these engine hard.

I am hoping to go back to a 3.73, or even 3.90, and do intend to adopt a A 833 trans for this.....

 

[JBNeal]

Gearing really depends on the engine's output...I had a '92 Dakota with the Magnum V6 5spd and a 3.23 rear axle, and the truck did fine all around and got over 21 mpg on the highway, running right around 2000 rpm...I swapped in a 3.55 Sure Grip, and the truck had a different character:  acceleration from stops was much faster, it seemed to pull better and had good passing power but seemed to run out of zip when ya got over 67 mph, fuel economy topped out at 20 mpg, running right around 2300 rpm, but it seemed more fun to drive because the engine seemed like it had more torque available at lower speeds.  So for that engine, 3.23 was good for cruising, 3.55 was good for hauling.  The flatheads are no different in that their relatively low horsepower and torque peaks under 2000 rpm so they do not benefit much from high revving like modern engines.  That's why these old beasts can pull decent with the tall gearing...I've had well over 2000# in the back of my 1ton, and that stock 230 with the 4.10 axle can get rolling no problem from a stop, but I don't get in too big of a hurry because I do not think those brakes are too effective at higher speeds.  I think the general consensus is that the flatheads can travel modern highways with the 3.73 and the 3.55, but the 3.73 pulls hills better than the 3.55.  The 4.10 axles that I am running on the 1/2 ton and 1ton do well on the mostly rural back roads that I travel, where topping 50 mph is usually done rolling down hill with a tail wind  

[Merle Coggins]

I'm happy with the 3.73 gears in my truck. With a rebuilt 218 and stock 4 speed I can cruise easy at 60-65. I can even push 70-75 when needed, although like Fred says, the engine is spinning hard at around 3400 RPM.

But, you must also consider tire size. I'm running 235/75R-15's at around 28" diameter. If you are running a 16" tire with a larger overall diameter that will change your overall speed:RPM ratios. I once did the calculations for Mark (ggdad1951) on FEF. With his larger 16" tires, and 4.10 gearing, he should only be spinning 100-200 RPM higher than my truck at any given speed.

Merle

[55 Fargo]

Yes I also am running P 235 75 15 tires, could certainly use smaller tires, but on a truck it don't look right.

I find my power band picks up at 2000 rpm, not where max torque begins at around 1600 rpm.

So with 3.23 gears, and when I shift into 4th at say 35-40 mph, my rpm is only at about 1600 rpm, but in 3rd much higher wanting a gear change.

With 3.23 diff gears, those older lower geared T5 trans gear spreads, might work.

My engine has miles, so with say a fresh 250/265, 3.23 gears would be okay, but 3.55 better...

Edited September 13, 2016 by Rockwood

[pflaming]

I have a 218, 3 speed, 235/75/15 tires, and 3:73. I easily run with the semi's on the freeways and can pass them at will. And I can drive up into the High Sierra Mts from 300' to 7000' in 30 miles, in third with a pulling engine but not lugging down and no heat increase. So I'm pleased. I will put on an electric radiator fan for reduced engine noise when I remove the doghouse and repair the under side fire damage. But I would not change because as is I'm confident I could pull a small fishing boat or single axle camping trailer with little difficulty. But cats are NOT allowed! 

[Jeff Balazs]

I have a 3.55 and to be honest it is a bit too high for this combo. It is fine in the flat stuff but lugs some in the hills. 3.73 would have been a better choice.

Jeff

[55 Fargo]

51 minutes ago, pflaming said:

I have a 218, 3 speed, 235/75/15 tires, and 3:73. I easily run with the semi's on the freeways and can pass them at will. And I can drive up into the High Sierra Mts from 300' to 7000' in 30 miles, in third with a pulling engine but not lugging down and no heat increase. So I'm pleased. I will put on an electric radiator fan for reduced engine noise when I remove the doghouse and repair the under side fire damage. But I would not change because as is I'm confident I could pull a small fishing boat or single axle camping trailer with little difficulty. But cats are NOT allowed! 

I know a gent with a 32 Chrysler 250 with well over a 100 k since he rebuilt it, 3.90 diff, T5 trans, 70 mph all day long, drove to BC on our west coast, Rockies.

He then hauled back a 2500 lb camper with it, no problem.

This is not just him talkin, I have driven in in this car, it moves nicely.

22 minutes ago, Jeff Balazs said:

I have a 3.55 and to be honest it is a bit too high for this combo. It is fine in the flat stuff but lugs some in the hills. 3.73 would have been a better choice.

Jeff

Yes your truck is heavier, your engine has miles, with a fresh 250, the 3.55s might be fine, but 3.73 would be better as Mr Flaming has posted, his truck is quite a bit lighter than yours too.

Edited September 13, 2016 by Rockwood

[59bisquik]

I currently have a 218 with the stock 3 speed. I changed my 4.10's for 3.73's and run a 225/60/17. She cruises at 60mph easily on the freeway at 2700 rpm and will pull all the grades with ease. 

I do have a R10 overdrive unit that I plan on installing before next April to help out with the long road trips and be able to cruise a bit faster. 

[John H.]

Where does one get a set of 3.73 gears for the 53 3/4 ton pickup? Thanks

[52b3b Joe]

On 9/13/2016 at 10:04 AM, Jeff Balazs said:

I have a 3.55 and to be honest it is a bit too high for this combo. It is fine in the flat stuff but lugs some in the hills. 3.73 would have been a better choice.

Jeff

 

I agree 100% with you on this. I have 3.54 ratio in mine, and it is sluggish taking off, and going up some grades are a challenge. My truck will struggle at 55 mph driving into heavy wind. I wish I had went with a 3.73, especially with pulling trailers every once in a while. 

[Jeff Balazs]

5 minutes ago, 52b3b Joe said:

 

I agree 100% with you on this. I have 3.54 ratio in mine, and it is sluggish taking off, and going up some grades are a challenge. My truck will struggle at 55 mph driving into heavy wind. I wish I had went with a 3.73, especially with pulling trailers every once in a while. 

Joe........yep. It is pretty quiet at speed though.

I may end up just going to a lower profile tire instead of changing the gearing. I don't know as I need to make up full difference in gearing. Now that I have a year on it it does not seem as far from optimum as it did to begin with.

Jeff

[pflaming]

The results on a 3:54 are important to me. I was thinking of going from my 3:73 down to a 3:55 but will NOT. And will retain the 3:55 in my suburban. which it has, as well. 

[Reg Evans]

Well, let me throw a wrench into the works.  I installed a 50 Chrysler 251 in my 52 1/2 ton with a stock 4 on the floor and  a 3.0 1965 T-Bird rear end.  The engine is stock and was rebuilt maybe 25,000 miles and 20 years ago.  I have no trouble going up hills or keeping up with traffic. Granny 1st is still a stump puller so I rarely use it.  I wouldn't change a thing now with this truck except maybe the noisy knobby treaded tires.

 

[Merle Coggins]

58 minutes ago, John H. said:

Where does one get a set of 3.73 gears for the 53 3/4 ton pickup? Thanks

John,

The Mopar cars of that era have differential assemblies that will swap into your axle. Mine came out of a '50 Plymouth. I'm told that there was an axle spline change in the early 50's that you may need to watch for, but other than that they are nearly a direct swap.

Merle

[Jeff Balazs]

2 hours ago, Reg Evans said:

Well, let me throw a wrench into the works.  I installed a 50 Chrysler 251 in my 52 1/2 ton with a stock 4 on the floor and  a 3.0 1965 T-Bird rear end.  The engine is stock and was rebuilt maybe 25,000 miles and 20 years ago.  I have no trouble going up hills or keeping up with traffic. Granny 1st is still a stump puller so I rarely use it.  I wouldn't change a thing now with this truck except maybe the noisy knobby treaded tires.

 

Reg;

I don't doubt you at all. But it is what it is. And my 4050# 3/4 ton FD with a sort of tired 230 ......there is just no way a 3.0 rear would work in it. A lighter truck with more displacement sure. I have to think that even a fresh 218 would be working hard to pull hills with anything higher than around a 3.66 rear. I could be wrong...but I would have to see it to believe it.

Jeff

[pflaming]

Jeff, he has a 251 engine, that is 1/6th larger in a lighter vehicle. That would haul a lot of wine! LOL

[Reg Evans]

Yeah, I should have said let me compare apples to oranges.

 

[Jeff Balazs]

Reg;

You were totally clear in what you were saying. And If we all had the exact same truck this would get boring pretty quick. One thing is for certain this isn't a one size fits all topic or answer.

Jeff

 

[Plymouthy Adams]

58 minutes ago, Jeff Balazs said:

Reg;

You were totally clear in what you were saying. And If we all had the exact same truck this would get boring pretty quick. One thing is for certain this isn't a one size fits all topic or answer.

Jeff

 

at one time it was....just look to your data specs

[55 Fargo]

5 hours ago, Reg Evans said:

Well, let me throw a wrench into the works.  I installed a 50 Chrysler 251 in my 52 1/2 ton with a stock 4 on the floor and  a 3.0 1965 T-Bird rear end.  The engine is stock and was rebuilt maybe 25,000 miles and 20 years ago.  I have no trouble going up hills or keeping up with traffic. Granny 1st is still a stump puller so I rarely use it.  I wouldn't change a thing now with this truck except maybe the noisy knobby treaded tires.

 

Reg, I have always found this interesting, your truck, with a 250 engine(most likely115-120hp), has no issues with a 3.0 gear diff.

This is great, and you have hills and traffic, so not like you are just putsing down a level old time highway on Sunday lookin for the ice cream stand.

Yur truck is lighter than jeffs for sure and any 3/4 ton variety, with the 250 engine, versus a 218, well the old saying "no replacement for displacement", rings true.

My truck older unrebuilt 228, but has duals intake/carbs and tru dual exhaust, with 3.23 gears. My new add-ons, make a big difference in getting my truck going, she is also a 108 inch WB lightest of the truck models, and I am happy about that fact.

Jeffs engine, while it runs nice, and has good compression, has not been rebuilt, and most likely has miles, these engines have a lot more juice once rebuilt, with a shaved head to raise compression.

Its a fact that these engines develop torque at a fairly low rpm, with stock cam specs, but the powerband is a higher rpm IMHO, not at 1600, but comes alive at 2500-3000 rpm.....

 

 

[Jeff Balazs]

13 hours ago, Plymouthy Adams said:

at one time it was....just look to your data specs

And for the most part they would all be too slow to drive much in todays traffic. Lot's has changed and has been changed over the years. And many of our trucks are not quite original anymore. Of all the mods one could make on one of these old flathead powered trucks this one (and brake upgrades) probably has the biggest effect on how useful the truck will actually be as time goes on. This topic is probably meaningless to the casual user or purist. But for those of us who really wish to get some serious use out of our trucks getting this right is a critical piece of the solution.

To me this is a very useful discussion. I am not concerned with going to car shows or originality to the last nut and bolt. Simply put my truck is a recycling project and I am trying to use as much of it as possible to build something that will get on down the road. And It is very close to what I was hoping for. A few more little tweaks and it will do everything I ask of it.

Jeff

[John H.]

Thank you for the responses. For the sake of clarity, I have the 218 and 4 speed in my 3/4 ton. What I am getting from your posts is that I need to hit the bone yards, eBay, or Craig's List for early 50's Mopar automobile rear end gearing? The brakes went out on "Old Blue" (my 4 year-old grand-daughter's name for this vehicle) on the way home from buying it, and Blue has been sitting up on jack stands for the past few months waiting for cooler weather to work on it. I have to repair a rear end fluid leak anyway and, after reading posts from you guys I thought I'd address the gearing as well. I brought the truck home on a 65 mph stretch of highway and had to pull over to the side a number of times to let the crowd behind me pass.

[JBNeal]

Duplicating Merle's setup is probably the most painless approach, but finding the correct differential unit is easier said than done.  Swapping rear axles to get a higher speed gets a little tricky; upgrading to a T5 overdrive transmission can be a challenge as well; doing both can be a major undertaking...then ya will need to address the front brakes...and so the snowball rolls down hill...for what it's worth, I put new tires on Dad's 01 CTD then headed up I-35, going the speed limit, and was being passed on the left and right for 20 miles, by vehicles bigger and smaller, so there are always gonna be leadfoots about, no matter what ya might be travelling in

[55 Fargo]

9 minutes ago, JBNeal said:

Duplicating Merle's setup is probably the most painless approach, but finding the correct differential unit is easier said than done.  Swapping rear axles to get a higher speed gets a little tricky; upgrading to a T5 overdrive transmission can be a challenge as well; doing both can be a major undertaking...then ya will need to address the front brakes...and so the snowball rolls down hill...for what it's worth, I put new tires on Dad's 01 CTD then headed up I-35, going the speed limit, and was being passed on the left and right for 20 miles, by vehicles bigger and smaller, so there are always gonna be leadfoots about, no matter what ya might be travelling in

Doing a complete diff swap is not overly difficult, and you get modern brakes to boot.

Doing a 3rd member swap is easy IMHO, but you gotta be able to pull the drums, backing plates axles to do the job.

T5 trans swap, all depends on what you want, some of the gear ratios aren't that desirable, with say a 3.90 or 4.11, 1st gear on the older NWC s is so low you will be starting in 2nd. This conversion is of course more involved as mentioned.

The Chrysler A833 swap, with a simple adapter plate would be easier, but again, this trans has no ebrake drum, like a T5, so something needs to be fabbed, or you need a modern rearend anyway.The gear spread is nice though.