Originally Posted by TeamButter
Right, the final drive ratio on a given car depends on how its drivetrain is set up, for example, in my Tamiya TRF 415MSX, its ratio(which I also refer to as the outdrve ratio, so as not confuse it with the final drive ratio, after adding the spur & pinion gears) is 2.187, which you get from taking the number of teeth in its diff pulleys(35) & dividing by the number of teeth in its center pulley(16). And for your TC4, it's most likely drived from the number of teeth in the diff gear divided by the teeth in the ring gear inside the gearboxes, & indeed its outdrive ratio is 2.5. So to get a final drive ratio, you take the teeth in the spur(72), divide it by the teeth in the pinion(say, 25) & multiply that by the outdrive ratio(2.5 in your case, which I believe would make the FDR about 7.2, if my math is correct). And just remember that the lower the FDR number, the higher your top speed will be, but acceleration will be slower....