[NoDOHC]

You will take about 0.2 seconds longer to reach 60 MPH at full throttle. (8.5 to 8.7 seconds).

If you never exceed 35 mph, you will see NO mileage gains at all. The tall gears only help you once you get into 5th gear. if you are shifting into 5th gear at 35mph in a stock ECU RX7 with 4.10 gears, you are probably hurting your mileage.

You will end up accelerating to a higher speed in each gear with the taller gears, so you will not save gas at all.

The 3.9:1 gears will give about 2 mpg highway, about 0 mpg city.

If you want better mileage:
Advance your timing (if it is NA - not a good idea with turbo)
Remove power steering
Remove air pump
Avoid revving past 3500 rpm
Avoid exceeding 30% throttle
Avoid excessive lugging (>10% throttle when below 1800 rpm)

I got 28 mpg highway and 24 city very consistently (in the summer) doing this with my stock ECU RX7. (Haltech would give 35/30 on the 6-port).