[Scuderia_Turini]

Hello Ian,

I believe that "motor offset" is simply about getting the motor low in the chassis so as to lower the centre of gravity. The lower the mass of the car is, the better it will stay in the slot.
The gear teeth of the pinion and crown need to "mesh" neatly or there will be excessive wear and noise. Using a zero offset gear set in a an offset alignment will waste power.  The gears are not so critical in a side-winder installation.

In off-the-shelf cars the motor shaft and driven axle are generally at the same height. In an offset configuration the motor shaft is usually below the axle line, but could be above.

Leo

Informative Diagram courtesy of Slot.it:-