JimC

Go to discounttire.com or tirerack.com and enter in your car information and they will give you a list of every tire they carry in the 235 55/17 size and you will see that the Pirelli that comes on the car from the factory says:

Pirelli Pzero Nero
Size P235/55ZR-17 98W FRD
Warranty 45,000 Miles
UTQG 400AA
Speed Rating Z*

But you could also put the Falken Ziex ZE-512 model on your car and the detail shows
Size 235/55R-17 99H B
Warranty 30,000 Miles
UTQG 360AA
Speed Rating H

That letter is only for that tire and what it is rated for. There are lots of different letters that could appear with the "R":

Speed Rating
The speed rating of a tire is based on U.S. Government standards for reaching and sustaining a specified speed. Typically, a tire with a higher speed rating results in better handling. Speed ratings are determined via laboratory tests that simulate road performance at various speeds.

Below is a list of speed ratings along with the corresponding speeds they represent. Remember that the speeds listed are test speeds and not recommended speeds. Speed ratings apply only to the tire itself, and not to a particular vehicle. Putting a tire rated for a certain speed on a vehicle does not mean that the vehicle can be safely operated at the tire's rated speed.

• Q Up to 100mph
• R Up to 106mph
• S Up to 112mph
• T Up to 118mph
• U Up to 124mph
• H Up to 130mph
• V Up to 149mph
• W Up to 168mph
• Y Up to 186mph
• Z 149mph and over

Now does the Pony Mustang require the Z rated tires? The speed limiter won't let you get to 149 MPH anyway.

rds1952

JimC,
I had an 89 5.0 with VR tires, so I am familiar with the speed ratings. When I bought
my 07, used a few months ago, they had just put on new tires in April. They put B.F. Goodrich P235/55/R17. I know size equals size and I'm ok. I'm just intriqued by the
conflicting info that I get from so called, reliable sources that says ZR17 came on the car originally and others say R17. I would just like to have closure on this not really important question. Once I bought the car, I downloaded several different 2007 Ford brochures online. I am going to look those over again to see what they say vs. my
replica window sticker that says it came with ZR17. I have found in the past, if I keep a car for a number of years, that the available info seems to be harder to find. I'm just
trying to get everything I can while my car is only 5 yrs old.
Thanks for you help.

BlackBetty

I'm guessing it's less of an issue of conflicting information and more of a shortcut from some of the sources to just not list the speed rating in the tire size. It's kind of a moot point in most circumstances because there's no way the car would've come with inadequately rated tires. In other words, most people just care about the size of the tire and people who do intend to go really, really fast aren't going to do it on the stock tires.

According to this article the current marking standards show that the speed rating should be indicated as part of the tire's load rating and not in front of the "R" as it was in the old days:

http://www.tirerack.com/tires/tirete....jsp?techid=35

Norm Peterson

There is a carcass construction detail - a cap ply - that as far as I know is universally present in tires beyond the 'H' rating (130 mph). It is frequently absent in tires of lower speed rating.

In tires for any sort of performance car, it's a really good thing to have, regardless of how fast you tend to drive.

http://www.sumobrain.com/patents/jp/...006002328.html


Norm

rds1952

TIRES AND WHEELS
(TYPE, SIZE)
Standard P215/65R16 96T
P235/55ZR17 98W

above from Ford Media Page for 2007 Mustang

This is a dead horse and I'm not losing any sleep over it.
The vehicle on the door frame has R17.
The Window Sticker has ZR17
When I bought the car it had 56,000 miles or so
and had new tires (R17) put on 3 months before I got it.
I also don't know why they would have put ZR tires on it
in the first place. While it's true Ford tried to (snazzy up?) the
V6 model, I don't know if they felt the need to go that far with ZR17.

Norm Peterson

The speed rating per se was probably a fortunate consequence rather than a primary reason. Given the other suspension upgrades from base V6, I'd think handling feel/behavior were more likely reasons.

Not everybody feels much difference among various tires of the same size.


Norm

danzcool

well, probably an economical thing on Ford's side since the 235/55ZR17 is the stock tire on the GT model, the painted bullit type 17" wheels are the base ones on the GT as well, so was probably cheaper and faster for them to pull from that stock rather than create another sku.

the Z speed rating is certainly not necessary since I believe the ECU for the V6 (2005-2009) limits the engine at 120MPH, and from what I understand there are issues with the stock V6 drive shaft above that speed also.

rls82959

OP, my door stickers say R17 and my window sticker says ZR17.

aaquib1992

I've got my stock pony pkg tires and they are ZR17 (if that helps clear anything up). I agree with danzcool, it was more a "parts sharing" then it was overkill. Cheaper to just use the regular GT 17in rim wheels then to go find brand new ones for the v6 package.

Either way I like them, but rain performance is kinda questionable while snow is just awful. If you really want stock tires then go for it, but you could do a lot better (i.e. continental extreme, or Michelin maybe?) for the price.

rds1952

aaquib1992,
I agree with those who say it was economical for Ford to use the same wheel and tire, as on the GT. I also agree, that they tried to "Pony UP" the V6 to make it more like it's more expensive brothers and sisters, GT, Shelby and Roush. So the tire offered better handling, etc... even if the speed rating wasn't going to be reached.

I've owned my share of Mustangs, 67-73-75-79-89-98 and now 07. If you have not figured it out yet, a Mustang is more like a pickup truck than a snowmobile. Lets see, a Mustang and a pickup truck are both front engine, heavy in the front and trunk/truck bed light in the back. Snowmobiles designed for snow, Mustangs designed for the open road. Mustangs and trucks have rwd in that no weight back end thus little grip. Snowmobile heavier in the back with tracks designed for the snow.

P.S. when you get a big snow can you find your "Snow Stang" or better yet can the snow plow guy find it. I'm now a fan of white, but it looks nice.
Maybe paint an American Flag waving in the breeze from hood to trunk....