Your current tires are the correct size (those mentioned may be 986S sizes?). I think I'd find another dealer if they don't know that!

I went with 245/40 and 265/40 Pilot Super Sports on my '06. The SS tires are fantastic, and the larger front has reduced understeer. I would not go to a 265/35 just to "correct" the speedometer reading.

Ok, thanks. This is just my summer toy car, already spent more than I wanted to on it, and decided to go with the Hankook V12's. I'd like to try out the SS, but I'll probably take 3-4 years to wear these out. Summer is short in North Dakota!

Sorry to sound stupid, but the 245's fit ok on the factory wheels?

Not a stupid question at all, they fit fine, though I wouldn't go any wider. An 8" wheel is the minimum recommended by Michelin for a 245/40-18. tirerack.com is a helpful resource for tire specs.