Using the '69 as the guinea pig, we'll show what 15-, 16- and 17-inch wheels look like on the front of a typical vintage Mustang. Using VWW wheels of various styles, we begin with a 15x7-inch Vintage 50 with a 37/8-inch backspacing. Essentially a Shelby 10-spoke copy, it looks great with the 225/60R15 tire (about the best size for this wheel). However, a 235/60R15 tire would also fit well. You could also stagger tire sizes by running a 225/60 or 235/60 up front and fit a 245/60 on the back. Although the 245/60 might be a little big for a wheel that's only 7 inches wide, it will fit and would look good. The Vintage 50 is also available in a 16x8-inch size with a 41/4-inch backspacing and would work great with the 16-inch-diameter tires mentioned earlier.
A 16x8-inch wheel with a 41/2-inch backspacing wrapped with a 225/50R16 (shown here) or 245/50R16 tire is a true no-hassle fit on the front of any '67-'73 Mustang. Use these dimensions with just about any make or model of wheel and tire and you can't go wrong. This one-piece 16x8-inch Vintage 45 wheel started it all for Vintage Wheel Works.
Now we're going big and it's really cool. This two-piece Vintage 45 in a 17x8-inch version with a 245/45R17 tire is the de facto setup to use in terms of size should you desire 17-inch wheels of any make and style. Fitment is perfect, fenderwell clearance isn't an issue, and you have plenty of rubber on the road--much more than any Mustang came with from the factory. Backspacing is 41/2 inches. A 255/45 or 255/40 tire will also fit here with the same size out back, or a larger 275/40 if you want to stagger tire sizes.
Moving to the back of the car, our current setup is the same 16x8-inch wheel as the 16-inch wheel shown earlier up front, but with a slightly larger 245/50R16 tire. This ideal combo looks great and won't cause any problems. On some cars, slight fender rolling might be required because some 245/50 tires are fairly big. You could have four tires that are the same size, but are different brands, so the tires will measure differently. For example, we've seen 245/50R16s on the back of two different '69 Mustangs and the tires were from different makers. One brand was clearly taller than the other, even though the size was labeled the same on each tire. Another option is the 255/50R16, which we've recently seen on the back of a '69 Mach 1. They are big, however, so some fender rolling will probably be required compared to running 245/50s.
If you need anything bigger than this out back, you can do it, but you'll likely have to contend with custom backspacing, which VWW offers. This 17x9.5-inch Vintage 45 wheel with a 51/2-inch backspacing and a 275/40R17 tire is downright huge. It's a nice fit, but VWW recommends staying with 17x8s out back. Outer fender clearance will be an issue on some cars, since all cars are different in this area. VWW offers several 17-inch wheel setups, including 17x7s, 17x8s, and these 17x9.5s. Custom backspacing is available on all sizes if needed. This package is about as far as we'd go. Beyond this, you'll likely get into custom sizing and backspacing issues, as well as tires rubbing on outer fender lips without considerable fender rolling.