Norm Peterson

That's only 0.1" difference in actual sidewall height, and since it seems you aren't going up to wider wheels I'm betting that the ride quality might even improve. You're gaining a little "bulge", which adds some vertical compliance (aka "give") when going over bumps and general road roughness.

The tiny effect on speedometer reading is irrelevant (neither the OE speedometer nor your ability to read it on the fly is accurate enough for a handful of tire revs per mile to matter in the slightest).



Norm

Mr. D

If ride is an issue, I'd still stick with at least a 50R. Unless competing in track racing I don't see any advantage to below the 50R except being in "style". In fact, I wish I could have gotten the 55R in a wider size like 250x55RX18.

What is the stock 2012 size - 235x50Rx18? I 'd stick with that, or go to a 250x55Rx18 (or wider) & change speedo settings. Cool guys won't agree, but Ford has a reason for the sizes they use on standard car. They work out better. Don't change, just to be changing. Just my 2¢!

JimC

MrD: You still get the size wrong on your comparison.
Try 235-50x18 compared to 255-45x18, and you didn't even use your suggestion in the comparison that you posted, the 250-50x18. So I don't know what the point is of your post.

The only one you got right is the 235-50x18, but your comparison are sizes that no one has suggested: 235-45x18 a full inch shorter than stock and a half inch shorter side wall, which you then compared to another size no one suggested, including you, a 245-50x18 taller than stock. So what is your point?

Norm Peterson

The 55-profile OE tires were 17's (235/55-17). A quick check at Tire Rack's site finds almost no support for 245/50-18. At 255/55-18, you're looking at 29" tall SUV tires, which on the first page of hits don't seem to be very highly regarded at all even in their SUV & crossover applications.

Seriously, 255/45's even on 18x9.5 wheels will give a decent enough ride, and would be better still on 18x8.5's or 18x8's. Goodyear's Eagle F1 Asymmetric All-Seasons have a TR survey rating of 8.4 (excellent) for ride quality, which I can personally vouch for, having those tires on one of my other cars (in a slightly smaller size but the sidewalls are about the same height as 255/45's). Tire construction affects ride quality perhaps as much as the widths of the wheels you mount the tires on until you go to extremes in wheel width or tire profile (say, 40 and under).


Norm

JimC

MrD, Try comparing a 235-50x18 to a 255-45x18 -- those are the options I am looking at.

Your 250-55x18 isn't an option, doesn't exist in the tire I am getting. Nor can I find on a search for tires by that size either. Probably why you couldn't find one.

JimC

Got the new tires on the car. They are the new Nitto 555G2, in the 255-45x18 size. Your butt dyno must be better calibrated MrD. because I don't feel what you described.

In fact I find these ride better than my Nitto Motivo in the 235-50x18. I will do a write up on them when I get a few more miles on them. I kept the Motivo for winter use, still enough tread on them and the 555G2 is the ultra high performance summer tire; not a good option here in Michigan when the snow starts to fly and temps get down to freezing and below.

Mr. D

OK, I see your point! My experience is a wider tire also helps ride. My comments were only on the same width tire with a lower radius. like a 235x50Rx18 compared to a 235x40Rx18. I'm sure you are right in the comparison you are considering. I guess being fussy about ride I'd be trying to use a 255x50Rx18 tire!

Do you know how wide a tire fits on the standard Ford GT 18" rim? A guy with a calibrated butt wants to know! I liked to put the widest 50R tire that will fit on that rim on my GT! With a supercharger I can sure use a wider tire, but I want a tire size I can rotate like maybe a 255x50Rx18.

Norm Peterson

Really need to think in terms of wider wheels, as 18x8.5's or the later-year 18x8's don't support running anything much wider than about 255 until you get to 275/50, which may only be available in drag radials and R-compound autocross tires (neither being at all suitable for unrestricted street use).

18x10's with about +42 offset should fit all around with up to at least 275/40 or 285/35, and probably 285/40 also working at both ends and rotate-able.


Ride quality, generally meaning a softer ride, needs the tire sidewalls to soak up pavement roughness and irregularities by changing shape to accommodate the roughness (maybe think in terms of the sidewall bulge bulging outward more as the tire is squashed into the pavement harder). Going wider with the tire without going wider with the wheel improves this ability (the static amount of bulge increases and equals greater vertical flexibility), but there are technical limits as to what is responsible. Greater sidewall flexing also means that more heat is generated, and it may affect a region of the tire not particularly intended to see maximum flexing and heating, and this is perhaps mainly responsible for the lower rim width limits. I've been through this off and on for about 15 years, starting when people on an other forum kept insisting on running a slightly out-of-spec combination (235/45-17 on 7" wide wheels, if it matters). And that was the gist of a phone conversation with a real tire company technical representative. Hope that helps a little.


Norm

Mr. D

See if I'm getting what you are saying:

1. Is the standard, base 18" wheel on a 2016 Mustang GT an 18x8?

2. A 255x50Rx18 is the largest that will fit on that wheel & can be, but you might not be able to find a "unrestricted road use" tire in that size?

Norm Peterson

Yes. Look about 3/4 of the way down on this page. Somewhere after 2008, Ford got a little stingy with the 18" wheel widths, as the 18" base wheels were 8.5" at least as late as 2008.



I looked for sizes wider than 255 that listed 8" as being OK and all I found at Tire Rack were drag radials and an autocross tire, neither of which is much good in the rain or intended for driving even in the dry that lasts any more than maybe a couple of minutes at a time. There is an industry standard involved, so it's not likely that only the tires not sold at TR are going to have all of the OK's for running wider tires on 8"

255 seems to be something of a "break point" for 8" wide wheels. In the profiles you'd typically choose for a sporty car - 45 and under in most cases - wider sizes than 255 need more than 8". 50 profile and higher tires for SUVs go a little wider, but you wouldn't be buying an SUV that came OE with 265/50 tires with much hope of having precise handling (and shouldn't want to choose its tires for a car that has decent handling or better).


Norm