[tseres]

also, manufacturing isn't perfect. we had this problem last year. what happens is 255 to one motor might only act like 230 (exaggeration) on another. one wyay to fix this is to individually slow down each motor to calibrate them:

pwmXX-=127;
pwmXX*=0.9 //for 90% power
pwmXX+=127;


the other (and preferred) way is to use sensors like (ideally) encoders, but you could use a gyro to make it go straight, or, dare i say it, gear tooth sensors as encoders (it only requires a slight modification to kevin's encoder code). the only problem is that GTS's have to be almost touching the gear to get a reading, and thereadings are usually pretty sketchy.

good luck!