wheel size Vs mpg

[andy-womble]

Just curious, but doesn't changing wheel diameter affect the speedometer and odometer readings?

Errrr yes.

Speedo/Odometer is calibrated on the following,
Final drive ratio and Rolling radius of the wheels.

In the good old days it used to be possible to swap the speedo drive gear at the gearbox end but don't know if people still do that

A lot of people do not seem to take this into consideration, also fitting different sized wheels could be viewed as "Modifications" by insurers and may invalidate your policy.