Thread: Calculator to find the right rear end gear for my car.

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One thing to keep in mind about figuring potential top speed, is that the important engine rpm is the peak power rpm, not the rev limit. For example, my LS3 has a peak power of 430 hp at about 6000 rpm, but it can be revved to 6500.

If you are performing a true top end run, most often you will either be over geared and not be able to reach the peak power engine speed before wind resistance limits the car's speed, or you'll be under geared and rev over the peak power engine speed. If you hit the rev limit, you're quite a bit under geared (too low).

With a modern overdrive 4 speed automatic, you should also figure the top speed in 3rd gear - usually the normal 1:1 ratio.

When I selected my gearing, I took into consideration the engine's rpm at a cruising speed of 70-80 mph, in the .7:1 overdrive 4th gear, in addition to theoretical top end in the 6000-6500 rpm range. With poor aerodynamics from a '37 style car, I might find that wind resistance limits my engine speed to far less than that. If not, it would be in the 160-165 mph range.