Gear ratio ?

[rpmgarage]

I have searched the threads, hoping not to bother with repetition of old topics, but can't find the opinion or answer I need...

Here goes:

I have a 1936 d2 Touring Sedan, with the original 3-speed trans (and a '41 Plymouth Flat-6). I am thinking of swapping the original rear with a Maverick, 2.92 gear ratio.

My questions are:

1. Is this ratio too high to work with this 3-speed? (opinions requested) I am running a 29" tall tire.

2. I am assuming the trans ratios are: 2.57 for 1st, 1.55 for 2nd, 1.0 for 3rd...PLEASE correct me and provide the correct ratios if I am wrong.

3. Using these numbers, I show to be turning about 2400 RPMS at 70mph (in 3rd), but only 870 RPMS at 10mph (in 1st). This sounds great at 70, but sounds like I will really have to rev it up to "get out of the hole".

4. Is this a common enough gear set to get my speedometer corrected with an external adapter? (And, where can I get one?)

I need to be able to make a continuous 90-mile interstate trip about once per month (and not get ran over in the process)...but 90% of my driving will be "town driving" at 45mph or below.

All opinions welcomed and considered.

Thanks, in advance, for everyone's help.

[greg g]

Think you might be happier with something in the 3.5 range. those 2 somethings were usually paired with auto, that got some help through the torque converter for low speed grunt, and starting on a hill would likely shorten you clutch life a lot.

[TodFitch]

That sounds way too tall for your 1941 201cu.in. engine. I wouldn't go taller than something in the mid to high 3s. 3.9 to 3.7 maybe but I don't know enough about what is out there to give a specific year/make/model recommendation.

If most of your driving is at 45 and if your 90 mile once a month trips are okay at 65 MPH, then your current rear end with its 4.11 ratio is probably fine.

Your 29" mounted tire diameter sounds a bit tall, but if that is accurate then you'll be turning over 3300 RPM at 69 MPH. Even at a more likely mounted tire diameter of 28" you'll be doing 67 MPH at 3300 RPM.

The Plymouth engine design was put through 50 hour tests at max BHP at 3600 RPM before the design was approved. So if it's not worn out it should run just fine at 3200 to 3300 RPM.

I've got a 4.375 rear end in my 1933 and if I'm trying to get somewhere in a reasonable amount of time run it between 60 and 65 which works out to between 3200 and 3400 RPM. And my little 190 cu.in. engine does not have the addition 8 years of refinement that your 1941 engine has.

See: http://www.ply33.com/Misc/speed

[rpmgarage]

Thanks, Tod.

The article is very informative and leads me to the following:

Maybe the investment isn't worth it then. I understand that the current combo will pull 15-17 mpg ("town driving") which would be 2nd best in class to my '65 Rambler (18.8mpg).

The stock rear end is great...just found that a PO plumbed the brake system with fuel line and wrapped all fittings with teflon tape! No wonder the car has been driven less than 5K in 20 years! But, the front and rear are fully rebuilt with new soft lines, white post restored cylinders, new shoes, springs, the works.

I guess that answers my question pretty quickly!

I am looking forward to all the other opinions (I know several of you have them, and they are welcome and considered before I make my final decision).

Thanks, again.

[1941Rick]

The 201 will be taxed to the hilt when starting out....your clutch will burn out quickly....

Like Tod said 3.91 or 4.10 would be best, especially if you are mostly driving city traffic.

I have the 3.91 in my 41 with a fresh 218 and it is great in town. Highway driving is helped with O/D. My books indicate the car came with 4.10 gears. Also said you do not have to use first to start out un less you are starting on a grade.....

[james curl]

The overdrive units are a 30% reduction. If you take 3.9:1 rear end ratio and multiply it by .70 you will get a 2.73:1 ratio which is fine on flat level ground at 70 MPH. Let some of the overdrive owners speak up about driving in overdrive without a kick down and in all gears. I have an overdrive in my 55 Chevy pick up with a 3.9:1 rear end ratio and I operate mine with a toggle switch straight to the solenoid without going through the governor. I can run mine in overdrive in all three gears from a stopped position. And I can tell you that 2.73:1 is too high for every day driving. I have a 3.55:1 rear in my 48 P15 which is fine, I think a 3.30:1 rear would work just fine as the engines hare very high torque at low speeds.

[Don Coatney]

I agree with what has already been said. Your suggested ratio will not work well for you. I have installed a 3.55/1 differential in my car along with an overdrive transmission. My 255 CI souped up Desoto engine pulls my car well with these ratios. But I have doubts your stock 41 engine would have enough grunt to get the job done with a similar setup.

[Gwellman]

I will try to avoid the gear ratio question, but offer some advice on correction your speedometer.

If you use original differential with OD, no correction is needed.

If you just change the read diff ratio, you may find it easier to just get a speedometer ratio adapter from a speedometer shop. There are also many places listed on-line. Basically, these mount on the transmissioin between the trans and the speedometer cable...usually this is pretty easy to do but check access first. Cost usually around $75.

[greg g]

I run my 46 with the 4.1 rear all around. It would like another gear for the highway but withthe calculators for fial drive and tire diameter on the web it tells me that 3200 Rpms is 62/3 mph. Which while soundig really busy by todays standards, has proven reliable on several long road trips at sustained highway speeds. So if I wee to change the rear end, I have a Cherokee with a 3.55 that would take the highway Rpms down a notch but would probably negatively effect my stunning around town zippiness.

If the bulk of your driving is don in town and at 45 to 50 you stock set up is fine with occasional 60+ mph jaunts.

You migh want to see what other ratios are available for your Furd rear annd see it they are easy to swap, and go with something in the mid 3's for you highspeed blasts.

[oldodge41]

I have a '41 Dodge with a stock '48 230 c.i. engine and three speed fluid drive tranny with 6.00-16 tires. I put a 3.27 ratio rear in place of my original 4.10. Low speed performance is fine. First gear is actually useful now. Drive-ability is better for the type of driving I do, secondary roads and primary two lane roads. Clutch wear should not be an issue on mine due to the fluid drive. What I have noticed is that I have to downshift on some hills that I didn't have to before. Could use a few more horses or tranny gears at this ratio. I had been considering swapping to a 3.55 when I found the 3.27. My speedo is way off at this point but I haven't driven the 3.27 rear enough to know if I will keep it or swap to 3.55, so I will live with the speedo till I decide. I know I don't want to go back to the 4.10 though. It is a much more relaxed feel on the road now. I once had a Plymouth duster with a 318 and three speed that had a 2.90 something rear in it and I could not keep clutches in it. My dad claimed it was the way I, a kid, drove it so he took it for awhile and soon had a chattering clutch as well. We swapped in an auto tranny since I didn't have a different rear. It was fine with the auto. That is my 2 cents.........Tim

[greg g]

The fluid drive does have a tendency to help out in the around town situation. Smoothing out the situations where a direct drive set up might be bogging down.

[Alshere59]

So what is the lowest we should go with a three speed with OD? 3.54? 3.73..

Assuming a 218 or 230 motor running fairly healthy.

Edited April 17, 2011 by Alshere59

[55 Fargo]

I have a good set of stock gears sitting around here for an OD set-up, 4.30, lots of bottom end, and the OD will cool things down over 50 mph...

[james curl]

I think something in the 3.30s range would be a good comprize. With my 3.55:1 rear I can take off from a red light in second gear and do if I need to get out of someones way quickly, as I can run to a higher speed in second before shifting. Low seems good for about 30 feet and 20 mph before shifting and second makes 40 almost as quick.

[Drdialtone]

I'm glad this topic is up. The previous owner of my 47 Club Coupe put a 9" Ford rear axle, 2.75. Runs great on the freeway, but steep hills is "running start" to get up as well as my steep driveway. Question is, can the gears in the differential be changed without having to replace the entire rear axle? I just recently spent a chunk of cash getting new axles and seals! Any ideas?

[james curl]

Yes the Ford nine inch has a removable center section that contains all of the gears. I believe any Ford nine inch center section will interchange with yours.

[Alshere59]

A couple of other questions for those that may know.

On a 1950 Plymouth P20 stock rear end what is the pinion angle with perches level and what is the distance from the center of the axle tube to the center of the U joint. I just bought a 1998 Jeep Cherokee rear and as it was by my family they picked it up. So roughly 250 miles away. The idea being if I can get them the measurements they will have it ready to slide under the car. I used the info from Blueskies site awhile back to get a drive shaft made. It shows the u joint is the same as a Cherokee. So if the distance from the center line of the u joint to the axle is close All I should need to do is bolt that part together. The rest is just getting the the pinion angle and shock mount welded on right. Oh and cut off all that extra metal. Anyone with with a 1950 Plymouth axle sitting around that could answer these questions?

Al

Edited April 19, 2011 by Alshere59

[Drdialtone]

James - thanks!! I'll go back to the differential shop that put the axles in and see what we can get. So, from the chats, something between 3.55 and 4.1 would be best it would seem.

[Drdialtone]

Oh, one more question... does anyone know where I would find a chart listing the Ford 9" options? Is new prohibitively expensive? Best to look for used? Thanks!

[Drdialtone]

YIKES! I had no idea there would be so many options!! Looking ring/pinion sets: 3.0, 3.25, 3.5, 3.7, 3.89, 4.11. Can anyone tell me changing from 2.75, and knowing that this is way too low for starting up a hill, which would be the best selection? I don't want to compromise the top end too much if I can help it, but 2.75 is way too low now. Is there much difference between 2.75 and 3.7? Better to go 3.89? Are these gears all I need or are there other parts required to complete the change? HELP!

[Young Ed]

I wouldn't go all the way to 3.7. I have a factory 3.73 in my 48 and it still runs pretty high RPM at 60-65. Since you are already making it around except for hills I'd jump to either 3.25 or 3.5. I've also read a change of less than .5 isn't worth the effort.

[james curl]

I would personally go with the 3.25 as I now have a 3.55 and would like a few less RPM at 70 MPH. I can now take off from rest in second gear with out slipping the clutch with the 3.55 rear gears.

[oldodge41]

I believe mine are 3.27 and seem to work well. Mine is coupled to fluid drive so I don't know if that makes much difference other than eating up hp and eliminating clutch slip. I think 1st gear is pretty low as I upshift pretty quickly. 2nd to third seems like a wide change but with the torque of these motors at low rpms I haven't had a problem pulling all but the steepest hills in 3rd gear. I just have to keep reminding myself this is not a 250hp motor so no gear is gonna make it act like one...................Tim

[Drdialtone]

Gentlemen, thanks for the advice. Sounds like 3.25 is the sweet spot. JT Diffs have new 9" 3.25 gears for $182. Is that reasonable? Thanks!!