Team Deathly - first trainer bot

[Deathly Hallows]

Today I got the motors wired up. The wheels look quite good, really solid things. Only issue is the bore is 13mm too big for any nuts I had lying around so I had to improvise and use some expanding cavity bolts. They don't stop them wobbling but allow me to see the spin. Wired it up all up my garage (it's freezing! can see my breath)

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Test run was OK. The Sabertooth mixer is doing the job in that its combining the channels for variable turns, opposite rotation for on spot circling, brake works well too. But the motor speeds seem to differ quite a bit. Thought it may be the gearing setting but they were both set on screwdriver and speed 1. When warmer and I have more light i will try a few more settings. But would appreciate any advice. Is this normal? Is it something I can fix on the TX? One thought I had was that one is running in reverse (compared to normal) and one is running forward. Could this be a factor?