[FChampOH]

Best fitment
front 8.5" +38 mm
rear 9.5" +48 mm

If you want a little more aggressive with the chance of rubbing,rolling fenders etc. add more width to either and lower the offset. For example:

front 9" +35
rear 10" +40

This info would be for either 19" or 20"

Tires: 245/40/19
275/35/19


245/35/20
275/30/20

Best setups to keep your rolling diameter in check....

 

[FChampOH]

We are splittin hairs here. The sizes that we recommended are 6 to 8 tenths of an inch bigger than stock tire diameter. As you said..not really enough to really matter. I have run 245/40/19 front and 275/35/19 rear on the IS-F without any issues. There might be a closer size than what we described but finding a decent manufacturer who makes the sizes can be an issue. A wider cross section on a front tire will pull around a bit more in the ruts in the road for sure but you will gain grip and crisper turn in. Seems like there is a trade off for most changes you make on a car.