I have a set of 18x8.5 rims, what is a perfect tire width for that size rim. I was thinking 255/35/18 but that's like top end for that width. Would a 245/40/18 be a better fit? Either way I know I will have to take it in to have the speedometer adjusted. When I got the car it had 225/45/18 on it. Zero traction

 

Whether you need to have the speedo corrected depends on the diameter of the tires (not the wheels) that you select, and what the PCM is currently programmed for.

Stock tires were either:

235/55-16 = 26.1"

245/50-16 = 25.7"

Which did your car have from the factory (see label on door jamb)? Is the program stock or has it previously been altered for a different tire diameter?

For the three tire sizes you mentioned:

225/45-18 = 26.0" - this tire fits on a wheel 7.0 - 8.5" wide

245/40-18 = 25.7" - this tire fits on a wheel 8.0 - 9.5" wide

255/35-18 = 25.0" (too small) - this tire fits on an 8.5 - 10.0" wide wheel

If you want the 255 width, you should consider:

255/40-18 = 26.0" - this tire fits on an 8.5 - 10.0" wide wheel

As long as you stay within the recommended wheel widths, the tire will function correctly.