[TheOtherGuy]

I didn't have any wheels on the motor. The shaft went straight into the encoder and nothing else. And you do have to remember that they are vex motors, which, I'm sorry to say, aren't the hightest quality motors out there in the market.

EDIT:
I tested 4 more motors using the same program. The motors were consistantly about 3% faster going backwards than forwards.

Motor 1
Forward: 10339
Backward: 10780

Motor 2
Forward: 10136
Backward: 10409

Motor 3
Forward: 9976
Backward: 10269

Motor 4
Forward: 10215
Backward: 10711

Motor 5
Forward: 10070
Backward: 10280