Did you modify the servos for continuous rotation yourself? In my experience I get this problem because when I was putting the pot back in it wasn't exactly the same on both servos. Pretty hard to do if you ask me.
In order to get the same speed I would find the center point (where the servo is completely stopped) for both servos and then use some math to make them spin at the same speed. For example if centerLeft = 30 and centerRight = 40, then you could define a variable like slow = 40 and use the command servo_left(centerLeft+slow) and servo_right(centerRight+slow) or whatever. Make sense?
I've never made the $50 robot so I could be completely off on this one
As far as I know you need to just test the speeds to work out how fast your bot will drive. It's easy to define some new variable like slow, med, and fast once you work out what speeds you like.