buenoshun

I just bought my new 2018 Mustang Ecoboost yesterday, and I want to replace the weheels to something nicer and better.
My first choice would be a Petrol P5A wheel, at a local shop, but it comes in the size 8x19, instead of the more common 8.5x19.
Is this an issue? This car for sure takes 7.5x17, 8x18, 8.5x19, 9x19 wheels. So it takes the diameter and width, but on all websites I looked at this specific combination of width and diameter is not listed.
https://www.americastire.com/buy-whe...ol-p5a/p/48524
The bolt pattern is the 5x114.3 that should work. What other parameters can prevent it from fitting?
Also, what tire size would be best for this 8x19 Petrol P5A wheel? 245/40R19 or 245/45R19 ? One is 1.8% larger diameter, the other is 1.8% smaller than the original 7.5x17 wheel that my car came with. The 255/40R19 seems too wide for the 8" wide wheel according to some websites.

Norm Peterson

Different offset from OE could either prevent it from fitting, or be too different from OE for an individual wheel supplier to give blanket acceptance to.

A 255/40-19 should be mounted on wheels that are between 8.5" and 10" wide, and that's a tire industry standard.

Don't worry about small changes in diameter if you're choosing all four new tires in the same size and the car originally came with same-size tires all around.


Norm

buenoshun

I see range for 8" is up to 245. So I ordered a 8.5" wheel instead from AM.
Ford delivers a version of the mustang (i think its the wheel and stripe package) that has 8.5" wide wheels and 255/40/R19 tires. Many websites say 8.5" wheels the max width is 255, so its okay. But some websites claim the width should be chosen in the middle of the range, and reject the edges of the range, aka only use 235 or 245 on 8.5". I'm not sure how hard rule this is.
Also AM has wheel+tire package with their 8.5" wheel and 255/40R19. I ordered their wheel (only) and a 255/40 tire in a separate tire shop (where they will do the assembly) and move my old TPMS into the new setup.
Maybe 245/45R19 would have been better, with the speedometer off by 1.8% in the wrong direction, that is "illegal". But I already ordered it

Norm Peterson

With respect to running tires on wheels anywhere from T&RA minimum-recommended to max-recommended, there are no "hard rules". Only individual preferences (and website information still amounts to that). Even the T&RA guidelines themselves are only guidelines, though because a lot of serious thought went into developing them they should be adhered to, certainly for general street-driving use.

It's probably better to consider that any given tire size is designed around the measuring width rim for that size. While this tends to be in the middle of the range, it's commonly not possible to be exactly in the middle (ranges tend to be 1.5" wide, and wheel widths generally come in 1/2" increments). At any rate, what the T&RA tables tell you is that there is a little tolerance either side of "measuring", which I suppose you could consider to be the "nominal design width".

So the simple answer that you've found some sites to be giving out is to stick to or very close to "measuring width". That's fine for those who are either not particularly fussy about ride quality or handling or who really want to keep the same balance among the various tire qualities that the tire was designed to provide.

Then you come to those who are not as fussy and those who intentionally put the various aspects of tire behavior in a different order. As rim width deviates from "measuring" in the narrower direction, you start trading away cornering response, steering precision, and handling in general in exchange for a slightly softer ride. In the other direction - wider than 'measuring' - you gain cornering & handling goodness at some expense in ride quality.


I haven't mentioned "appearance" as a reason for choosing tire size for any given wheel width (or choosing wheel widths for any given tire size), and I generally won't make any recommendations where aesthetics is top priority. Personally, I believe that the appearance should reflect whether the owner is biased toward cornering & handling or toward ride quality, at which point the setup will look right to him. What others might prefer should be irrelevant, as it's your car and how you want it to reflect what you prefer.

Just so you know, as a life-long corner-carver I tend to run my tire to wheel setups out close to or at "max recommended" once I've made the decision to choose a new set of wheels. Won't ever choose wheel widths at min-recommended for the anticipated tire size.




Norm

Agacerie

Good write up Norm, but I couldn't help but notice the L taillight of the poor and helpless buried ride in your garage. Makes me question your judgement ☺

Norm Peterson

That was a '66 C-Code coupe that belonged to my daughter, which had been heavily damaged (right front) in a crash some years ago. We'd hung onto it just in case either we developed enough interest to resto-mod it (too rough for much else IMO), or somebody else was willing to buy it. And a few months ago, somebody actually did buy it, even though by then it was minus most of the powertrain (somebody else got that not too long after the crash - a TBI 5.0 converted to itty-bitty 4-barrel carburetion and an AOD at that point instead of the original 289-2V and C4).

Now I can actually move around in the garage without having to turn sideways here and there, as it's really too small of a garage to be usable as such for anything bigger than a Pinto. Let alone be able to actually work on a car once you got it inside and the door closed.


Norm

buenoshun

At the end I bought a 8.5" wide wheel set from AM, and today they installed the 255/40/R19 tires. It does not look (to me) too wide, it is barely sticking out. As someone mentioned, 255 is in the (edge of the) range of 8.5" wheels.