Gear Ratio Calculations

Q: Gear Ratio Calculations

I was trying to figure out my differential gear ratio. So I did as suggested and marked the axle and the tire and counted revolutions. I came up with two sets of numbers where the marks both came back to the starting point at the same time.

Axle revolutions = 26
Tire revolutions = 19

Axle revolutions = 41
Tire revolutions = 30

If I did the math right the ratio is 1.37.?

What is this? How can it be so low(numerically). No wonder the car seems to be a bit of a dog out of the hole. Did I do this right? Can anyone explain why someone would knowingly do this to a 400?

Note: 1968 Firebird, 400, auto. Not the original rear-end (from external observations, specifically no code where everyone says there should be a two letter code)

A: If this is a non-posi rear (must be) then you should have seen the other tire turning the opposite direction to the one you were turning. In this case, the number you got is 1/2 of the numeric gear ratio, making yours not 1.37 but 2.74 which means it is probably 2.78 which is a standard Pontiac Ratio.

Why would someone do this? High top Speed? Good Mileage? Who knows?

A: First you need to 1 (spin the axel and count the yoke revolutions) and not 2 (axel and tire) if you ment the first sorry. But sounds like you looked at the second and that is why you got the reading you did….

A: Are you sure that the yoke and wheels came back to the “exact” mark? In this case, close doesn’t count. If it is a non-posi unit, did you block one side so it would not spin? Take another look.

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