Hello again everyone, I'm making another attempt to (hopefully) find accurate and reliable answers with this more recent thread. I own a 2007 Mustang GT and want to change out the factory wheels and replace them with a set of 19-inch or 20-inch wheels and a maybe a new set of Goodyear Eagles all the way around. I've been Googling the net for information on tire widths and general wheel specifications etc., but have been getting a lot of mixed information, with absolutely no definitive answers regarding MAX limits.
What I need to know when ordering a set of wheels is the following six (6) details for both front and back:
1. Best wheel diameter for daily driver car. 19s, 20s or other...
2. MAXIMUM SAFE BACKSPACING (So as not to interfere with inner fender-well or suspension components)
3. MAXIMUM OVERALL WHEEL WIDTH.
Measured across the entire wheel from the the inside flange to the outside flange.
4. WHEEL CENTER-LINE
Is the overall wheel width divided by 2.
5. OFFSET (Measured from the rim center-line to the inside face of the mounting hub)
Of the preferred (or optimal) offsets for the '07 Mustangs the specs I've heard bandied around are somewhere between 30mm and 45mm, but was hoping for something more specific and exact.
Another thing I'm a bit unsure of when referring to offset with respect to the newer Mustangs is how the offset is distinguished. In other words, is the offset for the '07s officially considered a positive offset or a negative offset?
I've googled and came up with some answers regarding offset. I've read that to get OFFSET you need to subtract wheel center-line from wheel backspace.
In other words: (wheel backspace - centerline) = offset.
- If backspace is less than wheel center-line the offset is negative.
- if backspace is greater than wheel center-line the offset is positive.
- if both backspace and center-line are the same then the offset is zero.
Another way I found is to simply look and notice whether the inside face/surface of the mounting hub (the part of the wheel that touches the brake rotor) sits further back from the rim center-line or in front of the rim center-line.
- If it sits in back of the rim center-line it's negative.
- If it sits in front of the rim center-line it's positive.
- If it sits flush with the rim center-line it's zero.
6. the ABSOLUTE widest tires that can safely be squeezed under the rear fenders into the wheel-wells WITHOUT any suspension or fender mods. In other words,what's the widest tire that will fit the factory setup? I want to avoid ANY rub... I don't want the slightest rub or scraping at all.
I've heard that people have been going all the way up to 315's in the rear with 20-inch wheels without any problems at all. I know for a fact that the factory Shelby's come with 285's in the rear directly from the dealer. Going with something wider than 285s in the rear would be awesome if it's possible. It would definitely give the car a much more "aggressive" look.
I not only want to put the widest tires/wheels all the way around on my car, I want to do it right without making any stupid mistakes.
I wrote down the tire specs from the stock Shelby I seen while at the cruise night some time ago. It had different wheels than what came stock, and the tire specs were as follows:
305/25/20's on the rear.
275/30/20's on the front.
I'm sure there is someone out there that did all the research and fitment. Hopefully they'll be nice enough to share the info. :-)
Seriously, I mean there's gotta be someone out there that's already figured out this info, right?
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Ed
Absolute widest wheels & tires revisited, 2007 Mustang GT 
