ratio
 

does anyone know/remember how to extrapolate final drive ratio to MPH ?

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.