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.
