so question...Has anyone played with this?
Here's a formula you can plug data in. The 5th gear ratio is .729 to 1, tire diameter is 27.03. If you know a specific RPM at a specific speed then you can calculate the effective rear end ratio. Guessed at some rpm/speed values and when plugged in it showed a fairly high rear end ratio which would coincide with mpg.
MOTOR - A Rear Axle Calculator