Thanks Ricosuave and Rmatt8748. The reason that I want taller gear is less RPM while on the freeways. And less RPM means better gas mieage, right? Thank you.

 

Good luck doing it, you would need to change the transmission ratio itself which is all custom work for this car.

 

I'm not sure you'll get necessarily get what you're looking for. Drag increases by the square of your speed, so while you might be able to cruise comfortably at 2500rpm around ~30mph, you may not have sufficient torque at the wheels to hold ~60mph. In addition, when you're using higher gear ratios, the same engine torque will produce a lower torque at the wheels. And don't forget even the AP2 doesn't posses the torque of a diesel below 3k rpm .

Even if you have enough power to cruise, you'll have difficulty going up hills or passing. Best case scenario you'll be frequently downshifting to 5th (which would be annoying considering it's now geared very differently from the taller 6th). This will kill your mileage even more, and increase clutch wear to an extent. Worst case scenario, you're put in a situation that calls for immediate defensive driving, and you can't accelerate in time.