[JimboJames1972]

This will apply equally to rears as well as fronts...

Larger diameters give more grip thanks to side wall flex when you load them up in the corners. With larger diameters you also get a bigger contact footprint and therefore more traction when accelerating in a straight line. However, larger diameters are more prone to pealing off the rims and chunking than smaller diameters are if the side wall flexes excessively. Drivers can also detect a slightly more sluggish reaction from the car with big tires.

The reverse is true for smaller diameter tires - less side wall to flex and a smaller contact footprint gives less grip and traction, but it can also make the car feel more responsive.

In general, run your fronts 1-2mm smaller in diameter than your rears (2mm smaller when the tires are larger and 1mm smaller as the tires wear down). Usually this gives a pretty good bench mark to work from.

James