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Sizing or fitting 18" wheels on 997 C4S

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Looking for a cheaper and easier way to have a set of track/DE tires for my 2006 997 C4S. Would like to get a second set of wheels, 18", to fit the rubber on, either street or DOT track tires.

Right now the car has Factory 19" Carrera Classic wheels. The manual says to fit the with:

235/35 8.5J with 55mm rim offset fronts; and

305/30 11.5J with 67 mm rim offset rears.

When I bought the car last year there were 235/35 front tires, 305/30 rear tires and I've been replacing with same.

The front wheels have on either side of the air nozzle 8 and 57, the rears have 11 and 51. The car was ordered with 5mm spacers as an option.

Just looking at the numbers above I would hazard a guess that the fronts are 8" and not 8.5" and have 2 mm more of an offset. The rears are 11" and not 11.5" as recommended in the manual with the 5 mm spacers used there for 56 mm offset instead of the 67 mm offset. Am I correct?

Is there a wheel fitment guide somewhere that is the equivalent of "how to read a sidewall of a tire" so I can decipher these numbers and get a set of 18" wheels that fit and won't adversely affect the handling?

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