im currently running 215/40/17 SO2s on my 01' RS. the recommended tire pressure for the stock tires r F31 and R29, but i dont think i should run the same pressure after i changed to 17". in that case what tire pressure should i run respectively? i believe the MAX PSI on the tires r 42psi. thx!

 

Stick with Subaru's recommendations for the lowest cold inflation pressure. If you want to add a little more responsiveness (at the expense of smooth ride), then maybe try 34/32.

 

If I were running 18s, it would be around 40f/37r. That's also what I run on my 17s. I do it for "wheel preservation" and gas mileage. The ride is more rough - you feel more of the road. There's no right or wrong; just stay between Subaru recommendations and max pressure listed on the tires.