The larger spur gears were really introduced for brushless motors, which just run better with really low ratios (11:1, 12:1 kind of ratios). The stock spur in the kit is the 104, and that should be a great starting point with a 10 turn motor. Here in the US I'd recommend a 17 tooth pinion with a 10 turn, but due to the different tires and surfaces (assuming you're in the UK) I'd really recommend whatever the other guys say.
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