[Perfectlap]

call the TireRack.com
They'll make it simple for you. You should be close to the car when you call so you can double check the sizes on the tire sidewall for the front an rear.

Also, its best for the good of the car to drive a good distance at least once a week, so that its more or less running for 30 minutes. If the engine sits for too long or if you only take trips that are very short its not really the best. Also, once you've been driving for 5-10 minutes, shift when the RPMs go above 3,000. Don't shift or drive the car long distances below 3,000 RPM.
Driving these cars to save on gas or so that it doesn't make a lot of noise will cause other problems down the line, these engines need to run in a spirited way to be healthy, it's not like a regular type car in that way. Try to avoid letting the engine idle for long periods, like don't start up and let it warm up for 5 or 10 minutes, that gets the engine hot fast and the car needs to be moving for the lubrication and cooling to do its job. Get in let it run for a minute and drive away.
Get the oil changed every six months even if you don't drive it too often. I personally recommend Castrol Edge 5W-40. Autozone run sales on it pretty often, works out to $5 a quart. you'll need at least 10. Find a good independent Porsche shop in your area (we have a sticky permanent thread on the General Discussions main page), that's if you want to avoid over-paying for basic stuff at the dealer. And wash the car yourself, avoid car wash places they'll end up swirling up the paint.