I have a 2003 ZX3 which I guess would have an MTX-75 right? When I drive 55 I get about 35 - 36 mpg regularly, but if I got 65, it drops to as low as 28. I'm wondering if I can put in a taller overdrive gear, and leave all the other gears the same. Maybe something as extreme as 0.5:1 or somewhere around there. Can that be done, and would it help improve gas mileage?