Here are some easy ways to measure your wheel gap -
Old-Fashioned Tape Measure
With the car on the ground, hang a piece of masking tape from your fender. Stick a quarter or a nickel on the end so that it hangs straight down. Next measure in from the masking tape to the outside edge of your tire. This will tell you how much space you have until the tire would contact the lip of the fender. Pick a spacer that is slightly less than this measurement. This will ensure the tire does not rub on the fender lip.
///8966[/ATTACH]
Even More Old-Fashioned Rulers
Same as the method above but if you don't have masking tape to hang from the fender, you can use a straight-edge or ruler and another ruler or tape measure. Gently hold the straight-edge from the bottom of the fender lip. Measure in to the outiside of the tire. The gap between the straight-edge and the tire is your available clearance. Choose a spacer size slightly less than this dimension.
///8967[/ATTACH]
Wheel Studs and Washers
For this method, you would purchase at least two wheel studs to thread into the hub and corresponding nuts to lock the wheel down (link to TMS 90mm studs). Remove your wheel and bolts. Thread the studs into the hub. Place washers down the stud to the hub. Slide the wheel over the studs. If it makes contact with the caliper or suspension, add more washers. Once the clearances are set, lock the wheel down with the nuts. Lower the car to the ground and roll - don't drive! - the car back and forth so the suspension will settle. Check your clearances again and also for tire-fender contact. Adjust if necessary. The end thickness of your washers will be your minimum spacer size. If you have space left over before the tire hits the fender, consider going with an even bigger spacer for improved looks.
Remember to remove your studs or install them the proper way before driving.
///8965[/ATTACH]
Studs and washers installed in the hub. Pictured is a BBS 90mm stud and 5 washers (roughly 15mm).
///8964[/ATTACH]
Studs and washers installed with the wheel.