I got a 4speed OD unit for my 70 Duster, What would be a good gear ratio for the 8.75 rear...mild upgrade, such as super six and exhaust, maybe cam...mostly street driven, rare highway driving....would something around 3.55 to 3.91 be alright? thanks in advance...Jerry

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73 Dart 4dr
70 duster
75 VW bug
ex-88 honda rice rocket
ex-70 cuda 383

Yep, 3.55s if you want lower RPM cruising, 3.91 if you can tolerate more. I'd probably go with 3.55s.

Lou

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Home of Slant6-powered fun machines since 1988

3.55s will give you an overall drive ratio of 2.49 in 4th gear (super cruise!)

3.91 will give you 2.74.

I've driven a car w/225-1bbl, 833OD and 3.23. It wasn't a rocket, but it wasn't objectionably slow, either, and got TERRIFIC highway mileage.

Me, I'd probably use 3.55s.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

depends on what your going for the 3.91's wouldnt be too bad for around town and "limited highway" use. and you'll definately appreciate the acceleration. but the 3.55's would be great cruising gears. I ran 3.23's with the 4sp o.d. and I didnt like the acceleration. but fuel mileage was awesome even with the 600 4brl + big cam. i'm currently running 4.30's and it does see limited highway use. gets about 15mpg on average. and is a great improvement for acceleration.

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Ryan Covalt
62 Plymouth Valiant Race Car 2170# 11.008 @ 117.65 MPH 1/4 Mile Best
72' Dart Swinger Street Car, 4 spd, 3330# 14.061 @96.58 1/4 mile Best

In your case I would go 3.91. I use 3.23 with OD for all highway driving.....but have the low gears in the 5 speed for acceleration. For strictly street 50> I would go 4.30 with OD....it would close up the ratios for less RPM drop between gears. Final drive with OD would be about the same as 3.23.

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Yeah....Im the one who destroyed this rare, vintage automobile.....