i now have a 2.73 rear and turn 1700rpm at 70mph, with a 3.73 rear i'd be turning about 2300 at 70mph. will my mileage decrease by a factor of (2.73/3.73)? or is mileage and engine speed not linearily proportional?

 

i doubt it would work out that nicely, it all depends on how efficiant the engine is at certain RPMs , you might actualy find that with slighly steeper gears youa actualy get better milage.

 

Aye, I agree. ^^^

When I lost 5th gear in my T-5, and was taking highway trips in 4th (direct ratio), I noticed that my car's MPG did not change. I don't remember exactly what 74 MPH was in overdrive (5th), but in 4th it's spot on 3 grand.

My guess would be that my car produces more torque at that RPM than at the other cruising RPM.

THEREFORE, less throttle is necessary to maintain the same speed.

Less throttle, more RPM, it basically balanced out.