[Joel J]

You can run both sides 2/3'rds full speed, and then record the speed of the left and right wheels. get the ratio of the two speeds, and use it to constantly descrease the output of the faster wheel.

For example, lets say the output of the right side is always about 80% of the left side when they are both commanded to the same speed. You'd then always scale the left side's output by 4/5'ths.

That's a rough scaling that teams did back in the days of the drill motors, and their winding bias. You said you are running some PID code, so that should account for the errors you didn't catch.

And again with that PID. If its not responding fast enough, then perhaps that's because you haven't limited the magnitude of the error before tuning? If you are using the PID to control velocity, then limiting the error may not do much good (you are already limited by the max speed of the robot).