[Kevan]

SlotIt gears seem the quietest, Scaleauto nylon gears run quietly also.

You can comfortably use AW spurs in SW pods and vice versa.

Gear ratio is only part of the calculation, tyre diameter can have a huge effect on gearing, compare a Mini to a pre-war Auto Union or Merc...rollout is what we're talking about, the distance the car moves for one revolution of the motor.  Typically for the same motor with the same 'rollout' the Mini would have a big pinion/small spur whilst the Auto Union would have a small pinion/big spur.