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identical motors, different speed

Hi all, I have 6 identical motors, or so I thought until I powered them all on at the same time for the first time and realised they all spin at different speeds.

Question:

Has anyone else observed similar behaviour either when using multiple identical motors, and/or when using the Arduino Motor Shield?  

Background:

I am using the Arduino Ethernet + Motor Shield rev3, with 3 motors connected in parallel to Channel A and 3 motors connected in parallel to Channel B. Each motor is connected to a wheel.

Wheel specialist

Hi, does anyone know of an online store specialising in wheels?

I mean really specialising in wheels?

I'm getting picky: I've been planning my robot for several months, and have calculated my exact requirements and I know exactly what I want - a wheel with tire, 80mm in diameter, must support approx 7.5kg in weight (so need to be quite wide) and I want gnarly/chunky off-road tyres, though my robot will be indoor-based, they must fit a 6mm motor shaft.