[Joe Johnson]

See if you can measure the current when the wheels are off the ground. You can also infer current by measuring the speed and backing out the motor speed then measure the motor current and doing some simple calcs to get motor current, but I prefer to know motor current directly.

Hey, I just had a brainstorm. How are you managing thrust on your wheels? Scrubbing the tires puts in a very high side load on the tires. If this side load is not managed well (for example if you let the tires rub against the frame) then that adds to the turning resistance that your motors have to overcome.

I bet that there is something going on there. Take some pictures and let us know.

Joe J.