Originally Posted By HotRodDave
Originally Posted By PurpleBeeper
This is over-simplified, but at a steady speed on the highway, just do a little math problem.

For example, if you got 10mpg with 4.10's and you swapped out to 3.23's, the you'd get about (4.10/3.23) x 10mpg = 12.7mpg traveling at the same speed. This should be really darn close.



I have actually found this to be close to a point, you can not keep adding higher ratio and just always keep getting better MPG as your engine will only make enough power to keep you moving at a certain RPM and if its always downshifting to get up the slightest incline you just reach a point of diminishing returns but if you have very deep gears already than this formula is kinda close. I have contemplated throwin a 2.20 rear diff in my daokta with a .69 OD ratio in the trans and really put it to the test but have not had the time or ambition.


This is true Dave. Another factor is that if your cam/engine makes peak torque (best mileage) right in the "sweet spot" of your highway rpm & you drop the gear ratio, then you may not be in that "sweet spot" anymore....but the math is still good for a ballpark idea of mileage, rmp at highway speed, etc.

70 Roadrunner convt. street car 440+6, NOS, 4-spd, SS springs '96 Mustang GT convt. street car '04 4.6 SOHC, NOS, auto, lowered "Officer, that button is for short on-ramps"