i have the stock 18"s on my coupe and the front tires are 225 45 18 and are still in greact contidition. my rears are bald and need to be replaced If i put 245 40 18's on the rrear would it make a big difference? Im asking because my friend has a set of 245 40 18's he will give me for dirt cheap. will it make my rear sag down more that the front of the car?

 

use the tire size calculator to give you your answers

 

This should answer your questions.
http://www.1010tires.com/TireSizeCal...?action=submit

 

The 245/40-18 will be too small for the back and will make it look uneven with the front being a 45 series. I would stick with a 45 series in the rear unless you are going to also change the front to a 40 series.

 

But don't u have to take in consideration that the front is 225 and the rear is 245?? 245 is wider and has a bigger sidewall when u have 45 series front and rear..

 

Thats how the car comes stock. The rear tires are slightly larger than the front.

 

I would actually recommend running the 245/40 with the 225/45

You will not have any issues what so ever.

 

Using the 40's on the rear will make the front tires taller than the rear. May have VDC issues with that. Best to stick wtih the oem sizes.

 

I blow out my rears a bought some used 245/45/19 and my vdc light come on after a 1/2 of mile..my front is 22/40/19...so like neal@tirerack said stick to oem

 

That would be because your rear tires were 1.6" taller than the front tires, or 5.8% taller.
You want to keep the diameter difference less than 3% to keep VDC happy

 

I run 225/45/18 fronts on a 8.5 wide rim and 245/40/18 on a 9.5 wide rear with no vdc issues.

 

thank you for the input guys. i went with the stock size.