[JimboJames1972]

Tire diameters have nearly as much impact on handling as the compound does.

Usually, a larger diameter will give more grip - a thicker sidewall will flex and deform as the tire is loaded which, in turn, will give a bigger tire contact patch and so more grip.

This is fine in low grip conditions, but many drivers prefer the more "direct" feel that smaller tires can give. This is why many drivers choose to skim their tires down to smaller diameters, especially if the grip levels are higher.

Lastly, most drivers consider that a car drives better with rear tires a millimetre or two bigger in diameter than the fronts.

When running standard Mardave rims I start my tires at 49mm diameter on the rear and 47mm diameter at the front for club running (low to medium grip). If I am running at the Nationals or at other very high grip tracks I use rears that are closer to 46mm and fronts as low as 44mm. I can go lower on both before I run out of physical ride height (i have the adjustable rear pod and use 1mm washers under the front late rather than nuts) but there is soooooooo little side wall left on the tires by this point that the car loses performance.

Hope this helps,

James