Up to this point in time, I have been shopping for 245/40/18's for the front of my car to run on 8.5x18's. Now I discover that 255/40/18 is actually much closer in diameter than the 245's.
225/45/18s are 25.9 in diameter.
245/40/18s are 25.6
255/40/18s are 26.0

Is anyone running 255's? Why is everyone running 245's?

 

"Why is everyone running 245's?"

good question.
however, the reasons i can think of...
1. tried and true. many have used this on this board without problem
2. better turn in
3. less drag
4. nismo recommended this combination 245/275 with their wheel package

"Is anyone running 255's". I am.

pp

 

I do too

 

greater availability. more companies offer 245 than 255, as i recall from my research. i'm running 255/40/18 up front and 275 in back, and so is jreiter, iirc.

 

+1 for 255/40/18 front, 275/40/18 rear. no issues execpt limited availability.

 

What he said. 255 front 275 rear (see sig). Very happy with this setup.

 

I run 255/40/18 Azenis RT-615 front and rear on four 18x8.5" Rays Track rims (made from 2 sets).

Fits fine up front and believe it or not it barely fits in the back with a H&R 25mm spacer.

Makes for a sweet stance and excellent balance (can rotate to).

 

If I run 255/40/18 and 275/40/18 does it matter on the measurments of the actual wheels?

 

yes. too wide a wheel, and your tire will be stretched; too narrow, and it'll look like a donut. each manufacturer has different tolerances for each tire.