Here are some constants. Given these you should be able to figure it using the following proceedure
1. Tranny ratio in 4th is 1:1.
2. Measure the diameter of the rear wheels.
3. Knowing the diameter of the wheels you can calculate the RPM of the rear axel at 50 MPH.
4. Read the tachometer at 50 MPH.
5. Assume a loss of 2% RPM thru the torque converter and subtract this from the tachometer reading. This is should be the drive shaft RPM with some small error.
6. You now know the driveshaft RPM and the axel RPM so figure the ratio using these two numbers.
However your question is confusing because the final drive ratio is a constant and does not vary with MPH.
Let me know what you come up with.
I got too many cars!! Insurance eats me alive. Dave
78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
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94 GMC Suburban - 90k
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