Ok, as the subject has shown up on several threads and somebody asked me if I’d had any issues running bigger wheels on the Mustang, I decided I would post some of my experiences.
I converted all the measures I took from metric to inches using an online converter. So, while I hope the numbers are accurate, there is no guaranty. (Remember: We Europeans use the metric system, which is why a Quarterpounder is called a “Royal with Cheese“!)
As for my car, it has been lowered about an inch using the shorter Mustangs Unlimited front springs (the blue ones) and rear leaf springs with a shallower arc.
The fenders are stock, no rolling or other alterations.
My Mustang originally came equipped with Mag 500 rims and F60x15 tires.
Now, I’d be glad if somebody could provide me with more input on this one, but I’ve been told that, converted to the new metric sizes, this is somewhere in between 225/60/15 and 235/60/15.
I used to have 255/60/15 on American Racing Torq Thrust rims with the standard -6 offset.
Suppose the above info is correct, then these tires were about 3,6 cm (1,3 inches) larger in diameter than the stock tires.
There were no problems with running those on all 4 wheels.
A while ago, I switched to 18 Inch OE Bullet wheels intended for late model Mustangs. 9x18 for the front wheels with a 255/45/18 tire and 10x18 rims on the rear axle with 285/40/18 tires.
These two sizes provide the same overall wheel diameter which is by the way the same as the one for the 255/60/15. (Again about 3,6 cm/1,3inches bigger than stock)
This is what it looks like on the car: EDIT: this particular picture was taken before the car was lowered, still with the bigger tires up front. Check further down for the update.
The rear 10x18 rims feature a 45mm offset and a 19cm (7,48 inch) backspacing. I use 45mm spacers on these rims, so I basically eliminated the offset.
So we can say we got a 0 offset wheel with a 13, 5cm (5,32inch) backspace. (19cm-45mm=13,5cm).
With this setup, the distance between the rear lip of the wheel and the leaf spring is 6cm (2,36 inch). The clearance to the steel mounting for the axle rebound bumper is a little smaller. Unfortunately I was not able to measure it exactly, but it was at least 1,6 inches from the wheel. Then there is always the possibility of trimming the outboard edge of it.
Based on this info, it is pretty safe to assume that even a rim with a 7 inch backspace could be used (Trimming of the edge of the rebound bumper mounting may be necessary).
Remember the fenders on my car have not been rolled. With rolled fenders, I guess that it should be possible to get a 12 inch rim in there. (That’s an estimate! Don’t nail me on that one!)
So much for the rear wheels.
Now, up front things look a little less optimistic.
Here I have a 9x18 wheel with a 35mm offset and a 16cm (6,3inch) backspace. The spacers are 40mm, giving us a -5 offset. So in theory we have -5 offset rim with 4,72 Inches of backspacing. (Again backspace of the rim minus spacer) .
With this setup, the wheel just barely clears the upper control arm. I could not measure it, but I could barely squeeze my little finger diagonally in between the wheel and the upper control arm.
It's a little hard to see on the pic, but you can see that the upper control arm protrudes into the space inside of the rim.
A 17 inch wheel with the same offset and backspacing would not fit!!!!
It would rub against the upper control arm, whereas the 18 inch rim clears it by fractions of an inch because of it’s greater diameter.
Even on the 18 Inch rim, the 4,72inch backspacing (with offset-5) is the max you can go, because again it would hit the upper part of the upper control arm.
If you use a wheel with no offset whatsoever, don’t go larger than 4,5 backspacing or make sure you got enough fender clearance on the outside, so you can put a spacer between the car and the wheel!
With this setup fender clearance is no problem. Remember the car is about 1 inch lower than stock and the fenders have not been rolled.
In order to lower the car further you'd have to roll the fenders or reduce the diameter of the front wheels by using a lower aspect ratio on the tires so they match the original size.
To me that was not an option, as I do not like the modern "low profile" look on vintage cars. I think this setup has enough rubber showing so it doesn't look like "Pimp My Ride".
On the rear axle there is no problem lowering the car, as there is enough room for it.
I hope this helps when you plan on going for bigger wheels.
Important: This is, what I measured on MY car with MY wheels. This information is only meant as a guide line and I will not guarantee that it is the same on all cars.
Same goes for the metric to inch conversion. As I said, I used an online converter and while I hope it converted correctly, please don't take it for granted! I posted the metric measurements, so, when in doubt, make your own conversions!