I just ordered 18x9 and 18x10 black Saleen rims and Im going with 275/45/18 in the front and I want to know what I should run in the back.. 315/45/18 or 305/45/18?

 

You don't want either of those sizes.

Front - 275/40/18.
Rear - 315/35/18 or 305/35/18.

A 315/45/18 would be way too tall and look stupid.

 

Why would you put a smaller sidewall on the back?? I cant find any tires that size. I can
only find 245/40/18 for the front and 275/35/18 for the rear. Im going with what tire rack recommended.

 

It won't be a smaller sidewall. The second number in a tire size is representative of the sidewall height, not a set measurement. It's called the aspect ratio. The sidewall height is the percentage of width.

275/40/18. The width is 275mm, and the sidewall height is whatever 40% of 275mm is.

315/35/18. The width is 315mm, and the sidewall height would be 35% of 315mm. If you did 40% of 315 you'd end up with a taller sidewall than the front tires, and it'll look stupid. Plus, the taller tire would affect your gear ratio and trip the ABS light.

I'm amazed how many times I've had to explain this, especially in this section. I figured this is basic knowledge. I should copy and paste this little "lesson" so I don't have to type it all out.

 

i'd recommend 275/35/18 front 285/35/18 rear
looks good, doesn't look over or under sized

 

I run 18x9 and 18x10. 265/35 fronts, 295/35 rears. Any wider in the rear is a waste and will hurt performance and that rear size will maintain your speedo/odometer.

 

Well I didnt know that the second number is a % and I even worked at a tire shop. Shows how much they taught me haha.

 

Are you serious?

 

Yeah I thought it was measured the same as the width of the tire.