Asked by: David Hunt, Redditch

A wheel is basically a simple machine converting torque - the force from the rotating axle - into the acceleration that drives you along. The mechanical advantage of any machine is the ratio of the force given out to that put in. The mechanical advantage of a wheel increases with circumference. That would suggest that the bigger the wheel, the greater the efficiency. But car designers have to balance that with other factors. While mechanical advantage increases with size, so does the torque needed to rotate the wheel. So excessively large wheels give you a mechanical disadvantage unless you have an incredibly powerful engine. Really large wheels are also heavy and raise the car's height and centre of gravity, disrupting handling and performance. That explains why most car wheels are roughly the same size - usually around 40cm in diameter.

Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.