i'm using a Wild Thumper controller on a DFRobot 4wd chassis and motors.
I noticed if I set a certain speed, there's a lot of difference between the left and right wheels.
But it is not consistent. The speed tends to equal out after a few seconds.
And it is dependent on the direction. e.g.: going forward starts with left going faster than right, and with going backwards it starts with right going faster than left... weird....
The effect is worse with higher speeds...