I dont understand why a smaller pully on back would be any differant than a bigger on front,by the time you are done the pinion is spinning the same speed.Your ratio is the same.RPM to tire speed wicth ever way you get there.But if you have any power at all,you may get belt slippage with the small pullys that you would not have with the bigger set up.Just remember if you can go bigger up front,do that then gear the back where you want it.Wont make your trans last any better,but you well have more belt traction.