Wait.. what? What tire size did u have before? Why didnt u buy the rims first then the tires to match up? Did u get a good deal? Are you stock height or lowered? Coils or springs?
Try Tirerack.com it may help u sort what rim size. I think some folks ran 245 on their cars. Depending on poke u want or tuck .. might need to roll those fenders
I cant remember my offset but i recall its pretty aggressive. Im on 235/45/18 on stock springs but they are aging. I get a good poke and rub only on big dips on the highway when i have a full tank of gas.
Is there any way you can return the tires because if you want to run a 245 with 18's it has to be a 245/35/18 to keep the stock tire diameter otherwise the tire will be a decent bit taller.