| Re: 18" Rims - Offset...and Tires?
Here is a previous quote from Ody that I saved when I was shopping for wheels/tires that gives an awesome explanation of tire sizing:
"The math goes like this: example - 245/45R18 : 245 is the tire width in millimeters. You can convert this to inches by dividing by 25.4mm/in (9.65" wide)
45 is the sidewall dimension in percent, with respect to the tire width. 9.65" (from the previous calculation) x .45 = 4.34".
Since there are two sidewall heights for any given wheel diameter (one touching the ground and the other on the top near the fender), you simply add two tirewall heights to the rim diameter (18" in this case). 4.34 + 4.34 + 18 = 26.69". This is the standard factory tire size for the CTS-V which is the same body as the V6 models. We know this fits fine with the factory V offset of 42mm. From experience, 40mm also fits, since it's <1/10" further toward the fenders. You can calculate the new height of different tires using the same formula, and comparing it (or simply use one of the on-line tire size calculators).
Offset, is the distance from the centerline of the wheel/tire to the lug mounting surface. 245's are the widest you're gonna fit on the front and not hit the upper A-arm pinch bolt on the inside, or the fenders on the outside (40-45mm offset generally is what's required). On the rear, if you want to go wider, you'll need to add all of the additional width to the inside of the wheel, since the outside is too near the fenderwell. Going to a 275 series tire, means an additional 30mm (compared to the example 245 series above). Adding 30mm to the inside only moves the centerline half of that amount, or 15mm. 40mm + 15mm = 55mm offset. One must measure very very carefully first, to make sure you have 30mm to spare before you rub the lower a-arm brace in the rear, since it's going to be very close. If it's gonna hit, then you'll need to shift it outward 5mm (to 50mm) or so and roll the fender seams. 275's are a tight fit, but it's been done several times. One needs to maintain the same sidewall height/tire diameter, which means going to a lower series tire (275/40R18).
Positive offset (bigger numbers) move the wheel inwards toward the body. Smaller positive numbers and negative offset (big lips) move the wheel outward toward the fenders. Most modern cars cannot use negative offset, since the wheels would be way outside of the fenders."
I have 18's and remember also in previous thread from Robert that if you run 245/45/18 your speedo will still be accurate because it will give you that 26" tire height that yer tryin to maintain. | 
