I am getting some new 18x7.5 rims soon and was wondering if this setup would be ok? I know many of you guys have 225/40 on your 18's but seems a little off according to the tire calculator.

 

Also, the tire calculator says the new tire is off by-2.3%. This means for every 1000 miles I travel I lose 23 miles on the speedometer right? Like actually traveling 1000 miles and having the car register 1023?

 

That's correct. Your registered speed will be higher than your actual speed. This tire due to the size being .3" shorter than stock.

 

....according to everything Ive checked, 235/40-18's are the PERFECT replacement for rolling distance for the stock tires. i.e. - 1000 miles stock = 1000 miles with the 235/40-18's. The only problem you might experience is possible rubbing at certain times depending upon your suspension and tire brand.

 

Indeed 235/40-18 are a better match to stock than the 225's. Several guys have been successful in mounting 235's with no rubbing whatsoever. However, a perfect match to stock it is not.