I am running the Michelin Gazelles on it, 2.50's on both.
I played a lot with 2nd gear starts and 1st gear starts this weekend, really watching the speed and motor response as it begins coming into a usable rpm band, then back calculating into rpm ranges. I do not think the stock sport 90 (or mine) can pull a 20-25% taller gear, which was my goal. the true fix is another gear in the transmission, which isn't going to happen. More rpm would work too, but delaying the intake closing will pull cylinder pressure out of the motor (torque decreased). I do custom camshafts and valve event calcs for a side business, and I have been really looking into the rotary valve mods. I am very hesitant with the age of the motor though.
so, I am going to begin with a 1 tooth larger front (~7%), then either tune with the back, or see if it will take a 2 tooth larger front (~14%). I am thinking 10-15% may be about the limit.
For the ones that have stock sport 90's, is 45mph about the best cruising rpm you can obtain too.