I'm going to get new tires for my v6, going from 17-inch to 18-inch. I'd like to get tire that are close to same height as the OEM tires that came on the car. I know it's going to be difficult to get an exact match, but would it be better to be taller or shorter than the OEM size?

If this matters, I'm also going to eventually get the eibach sport line spring set.

 

What year V6, otherwise stock height for my GT 2006 was 27.2 with 235/50/18's.

You can get wider rubber for the 18's with 255/45/18 tires all around at 27.0 inches height, that is a very good and common size for the Mustang and is a good fit for handling curves hard. Erik

 

It's a 2012 (I got it yesterday) with the 215/60R17 tires.

I'm more concerned about matching the current speedo calibration. The entire city of San Antonio is a freakin' speed trap. I was thinking smaller would be better so that if anything, and just found this web site that calculates speedo readings based on tire size:

http://www.csgnetwork.com/speedocalibcalc.html

(Maybe this should be turned into a sticky.)

 

I'm assuming you have the 8.5x18 stock Ford wheels now. Go with 235/50/18, as those are less then a tenth of one inch taller than your 17's. Erik

 

The rims are 18x9 (from American Muscle). I was considering 255/45R18.

 

This site is really good for comparing tire sizes : http://www.rimsntires.com/specs.jsp

Bigger tires will fill in the wheel gap a bit more, but be heavier, with a larger diameter effectively lowering your gearing slightly probably making you slightly slower.

 

255/45/18's will be 27.0" exactly, and any Ford dealer can adjust your tire rotations per minute to be at 27" on your tune for the speedometer. 255/45/18 will be real nice for launching your 305HP plus be great up front in hard cornering. Erik

 

Use any of the on-line tire size calculators for comparisons.