Electric:
48/15 - which according to "The Speed Calculator" is geared right on for 40mph (slightly less in fact). With your gearing, you have geared for more speed (47 will give higher top speed than 48, and so will a 16 over a 15). I think you would want a 15T pinion on there instead.
Jono83:
You can use mechanical brakes with 8th Electric. Basically you need an ESC where drag brakes can be turned to 0%, or completely off, and where braking force can also be reduced to 0 or off. Then use a servo + brake linkage set-up as normal. This servo is best used with 3rd channel mixing on your transmitter (where it plugs into the 3rd channel port on the Rx but acts off the 2nd Channel movements). This means that the ESC is not told to do anything other than 'stay at neutral' whilst the servo moves into braking and vice versa. This saves battery life.
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