Thread: rearend gears

Tweet

I have recently been through this calculation for a roadster with a mild SBC 350. You need to know what your rear tire size is, the gear ratios of the various gears in your transmission, whether you have a 1.0 high gear or OD and the ratio of the OD gear. In addition you need to know at what rpm your engine torque will be able to pull the car in OD to avoid constant downshifting from top gear. I found the following site to be very helpful once you have that data:

http://home.tampabay.rr.com/1bking/c...calculator.htm

I figured I would like to be in OD as low as 55 mph for mileage purposes and that the 700R4 I am using has a very low ratio for first gear so I need to cover fairly high rpm like 4500 if I wind it up in low gear. Fortunately the SBC 350 with my Crane street cam can easily run between 1500-4500 rpm. Then the last thing to consider is the stall rpm of your converter. It is no good to have an OD gear set up for 1800 rpm if you are using a high stall converter set to catch hold at 2500 rpm, if you see what I mean.
Play around with the calculator above and see what that tells you. I ended up choosing a 3.55 rear gear for a 2500 lb. roadster with the hope of good acceleration due to the very low ratio of the700R4 and hopefully (?) 20+ mpg with the 0.7 OD gear. As told above you also need to know the carb size because that will strongly effect your top rpm, assuming the distributor still works above 5500. In my case I just set up for the street, but the calculator above should work anyway to tell you what gear is matched for whatever rpm range you want.

Don Shillady
Retired Scientist/teen rodder