slade69

Oldsmobile Cutlass Supreme -69. 350 sbo originally with 2 barrel carb. What is my stock rear end ratio?

I'm planning a 4-speed conversion with overdrive and at the same time shorter ratio for rear end. Th2004r would be the easiest choice for a transmission. I still would like the overdrive to be "longer" than my 3rd now is. Need to find suitable rear end ratio.

joe_padavano

The base equipment rear axle ratio was 2.56:1 with the TH350. Of course, seven other ratios were optionally available. Unless you know for a fact that the car has the base ratio, you can't tell without finding the code or counting teeth.

Fun71

Or looking for the factory ID stamping on the perimeter of the ring gear, which is a bit easier than counting the teeth.

This shows 12 pinion gear teeth and 37 ring gear teeth so 37 / 12 = 3.08. From that I recall on my 2.56 rear, the stamping is 41 A 16.

bry593

Easier yet would be to find the two letter code stamped into right side axle tube.

The 200-4R won't like 2.56 gears due to the steep overdrive (.67). For example, .67*2.56 = 1 tire rev per 1.7 engine revs. At highway speeds, the engine is practically idling. The best idea is to change differential gear ratio to match the original 2.56 for highway cruising => 2.56/.67 = 3.82. The nearest aftermarket gear set is 3.90 for the "O" axle. This will enable tire frying launch torque but when on the highway, it's just like running 2.56 again!

Tuffnut

I have 3:42 gears with a 2004R over drive in my 75 Olds Supreme 350 . It really goes good off the line and cruises real nice in OD.


Jeff

slade69

Thank you all! This information really helps to proceed.