Fallstar01

This works with most 5 speed transmissions. In fourth gear at 70 mph, divide your rpms by 100. If your going 3270 rpms, you have 3.27 rear gears. 4th gear with 4.10 gears will have you at 4100 rpms which leaves you with 1700 till you redline (or 1900 more before your engine chokes). It's only an 830 rpm difference in that scenario. Since tuning your car (flash/chip/whatever) will often help your miles per gallon anyway, and since you need to tune to keep your speedo accurate with the new gears, I think you'll barely notice the difference (if any) to your gas consumption.
FS