overdrive help
 

I have decided to go with a Mitchell overdrive in my 37. Engine is in good health but it is a 21 stud 85hp stock engine. After doing the math I have calculated my rear end gear must be a 4.10 ratio. The rear tires are 29" tall. I am running just shy of 3000 rpm at 60. (about 2900)
My question is, and I appreciate any feedback.. Even though my cabriolet is a fairly light car, will my engine be enough for a 36% overdrive? That is what I ordered but I am thinking I may need to change my order to a 26% unit. Any feedback from anyone running a similar set up as far as stock engine and overdrive would be very helpful. Thanks in advance. All input appreciated.

Re: overdrive help
 

I have the 36% in my stock 85HP 39 coupe and it is a great match. I'm not sure what rear it has but I would guess a 3.78. I would stick with the 36%.

Re: overdrive help
 

The lower drop in rpm (36%) usually works better with the shorter ratio rear axles like 4.11:1 or 4.27:1 when a car is already equipped that way. If you have a 3.78:1 or taller gear set then the lesser 26% rpm drop would likely be a better fit. The drop in rpm is more like the drop in rpm between 2nd and 3rd gears. This blends the ratios better for overall use and still gives a decent rpm at modern highway speeds. 3.78:1 may go either way OK but 3.54:1 not so much.

Re: overdrive help
 

IMHO depends on the location. Flat and 36% is fine. Mountains and I'd go with 26%. I have 36% in both 36(37 motor) and 41 here near Houston.

Re: overdrive help
 

An original Columbia Overdrive is 28.5% just for a point of reference.




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Re: overdrive help
 

Thanks guys. I was second guessing myself after driving the car 50 miles or so yesterday. I will stay with the 36% Mitchell as ordered.

Re: overdrive help
 

I have the 36% and love it

Re: overdrive help
 

37 twenty one stud great eng

Re: overdrive help
 

I have a 60over 221 81a with a Mitchell 36%. I didn't build the engine to be a barn burner, but it has a few bores in the future and a decent cam. reground.

3:78 gears out back and it'll putt fine. Maybe at 70 some struggle, more with a head wind. I do feel it at the low end of the power band though, thinking about milling heads for a bit more compression. Not that I really care to go beyond 80, but it will do it, if i pressed. Low rpms at 70. Which over a long haul is better for the engine.

7:00 tires, Single carb , 3:78 rear, standard ignition rebuilt, fentons, 6volt etc.... totally rebuilt suspension (king pins, etc, tracks great), 40 hydro brake setup, 39 pedals.

I do like the fact that I can run second around town without shifting down, just use the syncro split. Flat lander and spur cut gear trans.




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