Sorry if I asked this one before. I just searched and couldn't find the answer that I should have written down.

I have the stock Pirellis now on 17" rims. If I go to 18" rims, I think there is a tire size that will approximately keep the total outer diameter of the tire the same as the Pirellis. That way I wouldn't have to mess with changing the speedo.

If you know that size, I'd appreciate hearing it again. I promise to write it down this time.

 

+1 on this question I have been wondering about rolling diameter.

 

235/50/ZR18's are the stock tire size on Stangs equipped with 18's.Same overall diameter as the stock 17's.

 

You can look on www.tirerack.com, a great tire resource. In answer to the question though, 235/50/18 is approximately the same rolling diameter as the 235/55/17's.

Edit:Poco beat me to it!

 

I'm guessing the stock Ford optional 18" ones are interchangable without a differance. I went from stock 17" Bullets to 18" Fan Blades, no ticks yet. I should check next time I see one of those speedometer checks on highway. 235/50/18 is the size on my Fan Blades. Any tire store should know how the two sizes compare. My 17" are stored so I don't know there size.
Sorry about repeating above. They were posted while I was picking out my post.

 

Most of the tire manufacturers have that tire dimension info available on their web site.

 

I believe stock tire height is about 27.0 to 27.2 in. You can do the old fashioned math version and calculate sidewall height from tread width to determine your overall height.

i.e. 235/55-17
235mm / 25.4 = ~ 9.25 in
55 ratio = 9.25 * 0.55 = ~5.1in

17 + 2 ( 5.1 ) = 27.2in

 

I had these options saved when I was looking for mine:

235/55-17 = 27.18"
235/50-18 = 27.25"
255/45-18 = 27.04"
265/45-18 = 27.39"
285/40-18 = 26.98"
295/40-18 = 27.29"

These sizes had the closest diameter to the stock 17's. They all had a speedo percentage error of less than 1 (actually .7 or below).

 

Does anybody make a 265/45-18, and if so, would it fit on the 8.5 GT/CS wheel?

 

Thanks guys. I will print out this info and save it. I may get a set of 18 inchers this summer.

 

http://www.miata.net/garage/tirecalc.html

 

I'll give it a try.

 

I think the biggest tire one can put on an 18" x 8.5" is a 255/45-18

 

It goes by revs per mile. If you can get the same diameter tire, the revs per mile will be the same, and you won't have to recalibrate.

CR

 

Yes it would fit but if I was you Id just go with 275/40s all the way around - which is what Steeda recommended to me...But I went with 295s although its on a 9.5" wide rim

 

I am going with the 275/40's on my stock 18" rims.

 

I am probably buying a set of Konig beyonds in 18 x 8.5.

So 275s all around would be ok??? And no issues on a lowered car?

 

You need a 9" rim I believe for 275s. Check TireRack.com

Steeda and Roush both run 275s all around with no problems. I mounted my Roush wheels with 275/40/18s on my Stang up front and didn't have any clearance issues. Now I just need an alignment and I can mount them on the car for good.