[Salik Syed]

we already have encoders mounted on both sides,
it should be fairly easy to do what you have described (measure RPM differences) , only thing i'm worried is that it is a non-linear change and simply putting a ratio of one wheel to the other may not work in all situations but I'm sure I can come up with some kind of simple algorithm based on 2-3 data points or someting.

For PID i am actually not doing velocity on each wheel, what I'm doing is just positional both wheels are to come to a target distance of 1000 encoder clicks.
what bugs me is that the drift is only when i drive forward and not backwards ! (?!)

EDIT: We are only running PID during autonomous, during regular driving drift is not a problem because our driver can compensate (its not bad in that the robot drives a half circle or anything, but enough that over 10 ft it goes 3 ft to the right during autonomous)