Arduino Motor Shield R3 running motors at different speeds

Hi guys and gals,

I’m in the process of making a small remote control tank and I’ve stumbled upon one slight issue. The issue is that although I’m telling the motor shield to run both motors at the same speed, it’s not . :frowning:

The parts I am using are below:

Arduino Mega 2560 (powered via usb)
USB Shield (powered via Arduino)
Motor Shield R3 (powered via external source)

I’ve rerouted the pins that would conflict between the two shields, and both motors run (just at different speeds!).

If anyone can suggest anything that might be helpful, I’d be really appreciative!