Model railroaders have struggled forever with this same problem. Getting two electric motors from the same manufacturer to run the same speed throughout voltage ranges is pretty much hit and miss. Add in slight variations to gearbox friction and you could be pulling your hair out in short order!

I would check for anything that might cause more friction in the side it pulls to. Short of that, unless there is the equivilant to the model railroaders solution (DCC control that equalizes starting voltage) you may just have to do the best you can. My cheapo tank does the exact same thing. It was so bad at first that it almost ran in circles! After looking for differences between the two sides and removing as much friction as possible, it still pulls to the left, but it is very slight.

Hope this helps some. []