What is the best wheel size for a Boxster that is autocrossed once per month and street-driven the rest of the time? Is it more difficult to find good autocross tires for 17" or 18" wheels? Should I abandon compromise and buy a set of wheels and tires for each use? If so, what's the best wheel/tire combination for autocross?

 

I would go with one set of wheels & tires for both. I use michelin sport cups on 18' turbo wheels and I street, track & autocross them. You will get a full years use on the tires but no more. In rain, your good to go to about the speed limit safely.

 

I guess it depends on the surface which your club AXes on. My stock Pilot Sports only lasted 4 events. So I switched to Falken Azenis Sports. Great tire for the $$. Lasts about the same time frame as the Michelin's, but at 1/2 the cost.

I still have my stock 17's, which seem to fair pretty well. A buddy of mine "upgraded" to 18's and it seemed his lap times slowed. Of course they were knock-offs and quite a bit heavier than factory 18's.

My personal advice: Run what you have for a season or two before you spend money on new wheels. Then when you'll have a better idea what improvements will give you the best bang for your buck. Who knows, maybe your lap times will improve more if you spend the $2-3K on the suspension.