Originally Posted by m4a1mustang

Stock fronts on the non-sport wheels are 225/50/18, which are 26.9" tall.

For 245, you'll want to run 245/45/18 which is 26.7" tall. The closest you will get to stock height.

For the rears, stock are 245/45/18, which are 26.7" tall.

For 275, you'll want to run 275/40/18 which is also 26.7" tall.

So your original specs were fine. These sizes will be perfect.