I apologize in advance if there is an absurdly obvious answer to this, but I have to ask. My stock tires, Pirelli PZero's, are 235/50/18, key here is the 50 profile sidewall. On both american muscle's site and late model restoration's site they are selling their aftermarket 18" wheels with a 40 profile sidewall in back, and a 35 in the front. Wouldn't the difference in overall diameter be an issue with the calibration of the speedometer and abs? The context here is that I was looking at aftermarket 18's so that I might retain the stock front tires and save a few bucks (cheap as hell, i know) but when I search tire rack for a 275/50/18 for the rear it says they don't have any. Any help for this noob would be greatly appreciated.

 

What you'll likely want to match is the tire circumference of the stock tire so it does not alter your gear ratio and speedometer calibration. Go to tiretrack.com and look up your tire size, then go to the specs tab. What you'll want to do is find the tire diameter - probably about 27.3" If you want a wider tire, you'll need a lower profile to keep the diameter about the same. Be sure to look at a few different brands because not all tire makers make the same size tires.

 

If you are using a tire that is 3/4" taller or shorter than factory, your speedometer will only be off by 1.83 mph at 70mph, and will only increase/decrease engine revs by 50rpm.

The factory 235/50/18 is 27.2" tall. If you wanted to use a wider tire on the stock 18" wheels and stay close to the original diameter, you need to use a 255/45/18 tire, which is 27.0" tall.

The 50 in your original tire is the "aspect ratio", which mean the tire sidewall height is approx 50% as tall as the tire width. As a general rule, when using the same diameter wheel, for every 20mm extra in tire width, you need to drop the aspect ratio by 5.

235/50/18 = 27.2"
255/45/18 = 27.0"
275/40/18 = 26.7"

The other thing you need to know about when using a specific width wheel, is how wide of a tire will fit. The factory wheel used on the 235/50/18 equipped Mustangs is 8" wide, and each tire size has a recommend min/max wheel width. The widest tire that will safely fit the factory 8" wheel is the 255/45/18.

235/50/18 = 6.5-8.5"
255/45/18 = 8-9.5"
275/40/18 = 9-11"

 

A slightly subject related question, is a Ford dealer able to re-flash PCM and adjust for different sizes?

 

Yes, mine offered to do it.... For $90. I have a tuner where I can just do it myself. Just would've been easier if they had done it is all

 

As hard as it is to resist I'm leaving our new car alone for a few years but did change tire sizes so I didn't have the super sticky r-compound GoodYears that came on my SVTPP equipped 500. They picked up every pebble/rock in the road and we have a lot of construction all over the area. No exact replacement sizes so went down one size and I know my speedo is a little off but I also wanted it correct so long term odometer reading was correct as well.

$85-$100 is what I've been quoted (basically an hour of labor) at my dealer. Taking it in tomorrow afternoon for the reflash.

 

thanks all, great info. @kn7671, totally changed my understanding of sizing, thanks for the advice.

 

I find this calculator real handy sometimes....

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

 

i've got both a 99 and an 01 miata, and i frequent that forum but i've never seen this awesome calculator. awesome find.

 

Lol that is crazy I have used for years...I actually used it when I put wheels and tires on my truck and was a truck forum.

 

Somebody just pointed me to that link and I came here with a question...lol. So my stocks are 235/50/18 as well and am looking at 255/40/18s replacements (on 18x9s rims, not stock), which are 1.3" shorter and the calculator tells me +4.5% on the speedo. I'd adjust the speedo but anyone see any other issues with this? I've heard within an inch of stock height. Also, looking at staggered next year, with 275/40/18s in the rear but the difference there is only .6" & +2.2%. What do you do with staggered, though, as far as speedo? Average the % difference, add them or what? Or do you just enter tire sizes on a tuner. Just sold mine for my old car so no tuner to look at right now.

 

I'm pretty sure rear tire size is the only ones that affect the speedo

 

Yeah, pretty sure youre right; thanks

Also, is there something you need to do to run staggered with ABS?