I just bought some used 18" wheels from a 2011 GT (the 5 spoke ones) to put on my 2012 V6. My stock 17" tires are 215-65-17 which are 28" outer diameter. The 235-50-18 that come on the 18" are almost 1" smaller diameter (27.25"). Does this make any difference? I have to put tires on these wheels, so I'm wondering, should I get a size that matches the diameter of the 17"s? That would mean I would need 235-55-18 that are 28.2" diameter. Does it make a big difference what side wall ratio (50 vs 55) I get?
You should be fine.
The tires on my 17" winter rims are 235-50-17 which are about 27.6".
I run 255-45-18 on my summer rims, which are about 27" . I check my speedometer with a Garmin GPS, they both read the correct mph. Here is a link for tires sizes and revs per mile that will take the work out of comparing tires. http://tire-size-conversion.com/tire-size-calculator/
If anything the smaller diameter tire will show a higher speed than you are actually going. 3/4" may be a negligible difference. It would theoretically rack up mileage quicker but again maybe not noticeably.
Location: PA to KY ('07) to IL ('09) to MS ('10) to FL ('11)
With a difference in tire O.D., you'll need a tuner to adjust the tire revs per mile so the speedo is on but it'll also change your overall drive ratio. It's probably better to stick with or as close to the OE tire O.D. Other than the minor change in rotational mass with the larger wheels, you shouldn't notice any performance change unless you DO change the drive ratio with a different tire O.D.
Think Nuke hit the nail on the head. I don't really want to deal with re-setting speedo, or different drive ratio. That's why I'm thinking the closest diameter in 18" is 235/55/18which is 28.2". My 215/65/17 is 28.0" OD. I have time to decide, my stock tires still have a few miles on them yet.