[Alan Anderson]

The elegant solution is to have feedback so you can maintain consistent wheel speeds on each side. Banner sensors pointed at gear teeth or at reflective marks on the wheels, or rotary shaft encoders, or magnets and reed switches, or any of a number of sensors can let your software measure the actual wheel speeds. You can have the code adjust the motor drive pwm numbers to keep the speeds the same.

The other "cheap" solution is to figure out what pwm values cause the two motors to run at the same speed, and hardcode them into your autonomous mode software.