REAR END RATIO vs MITCHELL OVERDRIVE

[Nick in Fayetteville]

What would be the preferred rear end gear ratio to use with the Mitchell Overdrive...Thanks

[Jim/GA]

The stock 3.78:1 rear gear is fine.

.

[Paul Bjarnason]

Originally Posted by Nick in Fayetteville

What would be the preferred rear end gear ratio to use with the Mitchell Overdrive...Thanks

Hi - It depends upon how much power your motor puts out -- I just went through deciding exactly that question. To make the decision I first spoke with Sue Mitchell and she said that if you have an essentially stock motor and the 3.78 rear end, the 26% overdrive ratio would work well. But, I just purchased a rebuilt motor from Schwalms which features a 6/1 compression ratio head, larger valves and a V-8 flywheel, plus I plan to put a Weber carburetor and headers on it. Ora Landis at Schwalms said he thought the motor I bought would pull the 36% overdrive ratio, if I were on an essentially flat highway, which is mostly what we have here in the Northern Virginia area. Also, I can always down shift for hills. If I don't like the performance of the 36% OD ratio, the cost of changing it to the 26% ratio is $400.

[Paul Bjarnason]

You didn't say if you were rebuilding the rear end. If you are not rebuilding the rear end or if you have a good ring and pinion gear, I would leave the stock ratio alone and just focus on deciding which of the mitchell ratios, 26% or 36%, you preferred. I was rebuilding my '29 Tudor's rear end and could have changed the ratio without any additional effort over and above what I was doing, anyway -- provided I were willing to spend the $700 or so on a new ring and pinion gearset. Also, somebody told me that if I have a good original ring and pinion, that if vastly preferred over a reproduction gear set. So, since I had a good used ring and pinion set, I decided to keep it.

[sphanna]

I have a Fordor 1930 One of the heavier cars. I have 3.78 rear end and a Mitchell 26% OD. A stock Model A with a 3.78 at 2200 rpm will give you a speed of 50 mph. With the OD 2200 rpm gives you 63 mph. I could cruise at 63-65 mph all day if I was so inclined (which I am not) without over reving the engine. I think it would be very foolish to exceed that speed in an A. Also you would be dropping out of OD a lot on many hills. Just not enough power with higher ratio than 3.78. I have HC head and B carb with expanded intake manifold which gives me a little more power. Stick with the 3.78 I would. A 36% OD is just too much for a 3.78

If you go with the 4.11 gearing which is also good, and the 26% OD and 2200 rpm you will cruise at 58 mph. Straight 3.78 = 50 mph. If you like to cruise at 50, no need for OD. If you will be driving 60 for cruise I like the 4.11. If you want to cruise up to 65 then 3.78 and 26% OD. My opinion is free and worth every penny. Only thing I know is that my numbers are correct.

[TerryH]

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I agree with the 3.78, and past experience has shown most prefer that. You didn’t mention which Model A you have.....I also have the heavier Town Sedan, and the same time I added my Mitchell, I had the 5.5 HC Snyder head installed. To me, it is the perfect combo....I can still pull modest hills in OD, and still cruise effortlessly at 60 mph. I use mine more like a 4th gear instead of an OD.....even on surface streets, when I hit 40 mph, I shift into OD. Part of the fun with the Mitchell is using it like that.....at 40-45 mph the car is so quiet and smooth....it is a revelation!

[daveymc29]

With the mitchell you end up with six speeds forward and two reverse. I have a 3:78 and wish I had used the 4:11 for climbing the hills. Could drop to second direct and pul most anything and I'm not much interested in 65 all day as some say, though I have done it I ended up getting the engine rebuilt soon after. Some of our hills go for 7000 feet or more so one could use a lower gear and patience to reach the top. 8000 is about the highest I have been and it worked real well, often in 2nd over and some in 2nd direct, which isn't real fast after an hour of tooling along, but it will get you there,

[GerryAllen]

Due, to having to replace my rear end I installed a 3.54 and have found it brings my engine RPM down, which one would expect. I am quite happy with it and enjoy the drive.
I went the 3.54 with the thought going to a Mitchell overdrive at a later date. What are your thoughts on doing this? Is an overdrive necessary with a 3.54, 95% of my driving is done on relatively flat roads with the odd hill.
Your thoughts would be appreciated.
Gerry Birch Bay WA

[BRENT in 10-uh-C]

One thing to consider is, do you typically only drive/tour by yourself with your Model-A, --or with a group of Model-As? From my experiences, you want to tour with ppl that have vehicles that are similarly prepared. If you have a faster gear ratio, you will find yourself creeping away from them causing them to speed up to catch up, -and always shifting at different times. If you are a 'lone wolf', then it really boils down to your vehicle's engine power and your normal driving terrain.

[30 Closed Cab PU]

Originally Posted by GerryAllen

Due, to having to replace my rear end I installed a 3.54 and have found it brings my engine RPM down, which one would expect. I am quite happy with it and enjoy the drive.
I went the 3.54 with the thought going to a Mitchell overdrive at a later date. What are your thoughts on doing this? Is an overdrive necessary with a 3.54, 95% of my driving is done on relatively flat roads with the odd hill.
Your thoughts would be appreciated.
Gerry Birch Bay WA

A couple of previous threads with some info. Try a search on differential, and perhaps overdrive, possibly more info. Hope this helps.


https://www.fordbarn.com/forum/showt...t=differential
https://www.fordbarn.com/forum/showt...t=differential