Mitchell Over Drive 26% or 36%

jboggess

jboggess

Senior Member
I noticed that there are 2 types of Mitchell Over drives that you can purchase from the Mitchell web site,,, a 26% or a 36%,,, i wonder what would be best with a standard 3:78 ratio rear end....
 
I believe which ratio you choose depends on the type of driving you do (hills vs flat, slower back roads vs highway, etc). I have a 3.78:1 rear with the 26% Mitchell and it seems to work fine for me. Others here will probably chip in with their experiences.
 
jboggess,
I just installed the 26% Mitchell and think it's great. I'm in Florida were it's very flat and intend to drive longer distances to shows. It really does quiet down the engine and I can easily cruise at 50mph. The 36% would have been too much for my original engine as accelerating in OD is slow. Very happy with the 26% unit.
I've also driven a coupe in Maine with the 26% Mitchell and lots of hills and it is the right choice there also.
Al Leach
 
26% with what you have. I have that in a roadster with a 3:54 diff. Works as the car is so light. Mitchell told me that the ideal was 4:11 with the 36. I have the 36% B/W in a coupe with the 3:78. That will not be the car I take to Sparks. It is okay on occasional hills, but from the Bay area to Spark is almost 8K feet climb on the route we take, Hwy 88. That is too much for my engine. I had a 3:78 and 36% B/W in a ccpu with Cragar overhead. That worked will over the mountains from here to Portland and back. IMHO your choice should be the 26%. Great product, you'll like it.
 
I have a 29 tudor with a Bromfield 5.9 head, Stipe IB 30 cam, and 3.54 rear. The 26% Mitchell OD works great out in the mountains of VA, West VA and PA.
 
I have run a 26% Mitchell with 4:11 in my Phaeton for over 90,000 miles. Really like the combination whether running flat land or mountains. Great product. Zero issues with the overdrive.
Good luck with what you choose.
 
My phaeton has a 3.78 diff and a 26% O/D. I have done about 30,000 miles towing a camper trailer behind it. We travel at 50 mph on mostly flattish country (Australia is that way!). The camper weighs about the same as the car. While acceleration doesn’t make your face go baggy, the car has done it well.
Oh yeh, the engine has a 5.5 head, downdraught carby, counterbalanced crank,inserts and slightly modified cam.
In a nutshell, I think in a car with a lighter body, the 36% O/D would be fine if you have an improved engine. With a standard engine, I’d stick to 26%. Even nothing changed but a high compression head and you’ll be fine.
All that said, don’t expect your fuel consumption to improve. It takes the same amount of fuel to go (say) 50 mpg whether with or without an O/D but things sure run quieter.
 
Purdy Swoft said:
If a completely stock model A won't cruise at 50 MPH , something is wrong .
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My Coupe may do 50 mph but I don’t wish to push it that hard. Before doing much distance driving, I will need to do something to reduce the rpm. Perhaps it’s mostly a mental thing.
 
My recommendation would be to install a 26% Mitchell overdrive with either a 376 or a 354 ring & pinion. Back in the days when the Borg Warner overdrive was installed in many Model A's, they were 33% and owners soon learned by experience that the ratio was too high and it could only be used above 50 or it would lug the engine. Many opted to install a 411 ring & pinion, which was better suited.


My Victoria has a 23% Ryan overdrive with a 378 and I like that even better.


Tom Endy
 
kd4lok said:
My Coupe may do 50 mph but I don’t wish to push it that hard. Before doing much distance driving, I will need to do something to reduce the rpm. Perhaps it’s mostly a mental thing.
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I would be more concerned about lugging the engine . Lugging the engine with low RPMs in high gear has ruined the bearings in lots of model A engines . As strange as it may seem, sometimes more damage can be done trying to take it easy on the engine than running at full throttle . Good luck and no harm meant .
 
Thanks for the comments guys, I don't wish to go 50 at all, my engine is new, sounds, runs great but i cant even go over 35 with out the engine sounding like its going to blow up the RPMs are sky high,,, is it possible that i have a wrong transmission or something, i believe Model A's only had one transmission
 
jboggess said:
Thanks for the comments guys, I don't wish to go 50 at all, my engine is new, sounds, runs great but i cant even go over 35 with out the engine sounding like its going to blow up the RPMs are sky high,,, is it possible that i have a wrong transmission or something, i believe Model A's only had one transmission
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That doesn’t sound right. Have you tried top gear?:p
Seriously, if you do a search here, I’m sure you will find info on how to work out what diff ratio you have. That might explain a lot.
 
If your engine is "sky High" over 35 mph or even 50 rpm, something is really amiss. Your rearer end can not be the standard 3.78. At 35 mph with a 3.78 reared ratio, your rpm would only be about 1550. At 50 mph it would be at 2200 rpm which is not overspeeding the engine. With a 26 % overdrive the 2200 rpm will yield 63 mph. A little fast driving maybe but 55-60 mph with the 26% is a very nice easy cruise from for the engine. And it is quiet.as the rpm is about 1900 - 2100 rpm.
 
I hope this comes out in a readable format. These are the RPM and MPH for my car at various RPM with the 3.78 ratio and the Mitchell OD 26%. I have 21 inch wheels. Hope this helps.

1000 3.78 22.79
1500 3.78 34.19
1750 3.78 39.89
2000 3.78 45.59

1000 2.8 30.77
1500 2.8 46.16
1750 2.8 53.85
2000 2.8 61.55
 
We used to go to Vegas every year for the 'Winfield weekend'. My wide-bed has a Colombia rear, 26% OD. The first few years I had 31" rear tires, and it did OK. I later switched to 28" tires, the truck pulled the hills better, ran cooler, and got better gas mileage!
 
If engine is noisy and shaky I would look at the balance of the fan especially if it an old steel 4 blade ... they are heavy and really shake the whole car if not well balanced.

Try one of the plastic 5 blade fans if for no other reason than to check for unbalanced.

Plastic blades do less damage to hood and radiator if a blade breaks off.

Although I have never seen a plastic blade break off.
 
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Benson said:
If engine is noisy and shaky I would look at the balance of the fan especially if it an old steel 4 blade ... they are heavy and really shake the whole car if not well balanced.

Try one of the plastic 5 blade fans if for no other reason than to check for unbalanced.
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Yes, the fan is o good place to start. You said your engine is new but how well was it balanced by the person/company who rebuilt it? How reputable are they? Sounds like you might need to talk to him.
 
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