Tires: OK to switch tire sizes on a car?, sara sara, rear tires

Question
Hi Brett,
OK, here goes.  I have a RWD 2007 Lexus IS 250 and it's time for tires.  I have 225/40R18's on the front, which is no problem, but I have 255/40R18's on the rear tires.  I was quoted a terrific price at one place, but I later learned that he would be ordering 245/45R18's for the rear tires instead of the current size I have.  Is this OK?  Will it make a difference?  It's such a great price, but I don't want to do anything that would harm the car.  Of course, he says it makes no difference, but I wouldn't expect anything less.  Please help!
Thanks!
Sara

Answer
Sara,

 I don't blame you for your concern.  A P245/45R18 is the same overall diameter as the P255/40R18 and should not present any problems.  My concern about the great price would be to assure that the speed ratings are the same on both tire sizes.  If they are then no problem.  If they differ then your ride and handling could both  be affected.