I have a 1SF with 3.23 gears and 17" wheels. The tach reads exactly 2000 RPM at 60 MPH. I was thinking about going to 18" wheels with 245/50R18 tires to drop the RPM and possibly improve gas mileage. Does anyone know how much this will drop the RPM?
I know the 1SG has 18" wheels, but with 3.42 gears so does the 1SG option run about the same RPM at 60 as mine?

Here is a handy calculator to get your overall diameter for tire and wheel.
I have the 1SF pkg with the optional QF8 18 in wheels. Try stay within 3% to minimize any errors you may have with your various computers/speedo.

http://www.1010tires.com/TireSizeCalculator.asp