This illustration should help...
This illustration should help clarify some of the wheel terminology we've been talking about, like that pesky offset stuff. (Courtesy Vintage Wheel Works)
Going up from, say, stock 14-inch rims to 16s would be an example of Plus-2 sizing, and would require a tire having a significantly shorter sidewall (lower aspect ratio) to maintain a similar overall diameter. So, if your Mustang now has F70x14 rubber (roughly P215/70R14 in modern-speak), you'd have to go with something like a P225/55R16 or P245/50R16 tire to get close to the same overall diameter and circumference when using a 16-inch rim. A tire's "aspect ratio" (the second number in its size designation) describes its sidewall height as a percentage of tread width and, for any given overall diameter, a tire with a lower aspect ratio will also be wider, which in turn may require a wider rim width for proper bead mounting. In our modified Mustang world, wider is often better-but only if everything fits.
The key to a successful marriage of custom wheel and Mustang is pre-purchase measurement, measurement, measurement. Measure with the suspension at normal ride height (not fully drooped), and, up front, it's important to measure with the wheels pointing straight ahead, as well as with them turned to full lock in both directions. Many wheel manufacturers and vendors now want you to supply specific measurements of your car's brake hardware as well as clearances to chassis/suspension hardware, wheelhousings, and so on, front and rear, when ordering their rims. Such pre-order scrutiny is a very good thing for all parties involved.
The amount of backspacing...
The amount of backspacing is easily measured by placing a straight-edge across the inside edge of a rim, and then with a tape measure or ruler measuring to the wheel mounting surface. The greater the backspacing, the more inboard the wheel and tire are positioned. Offset is not directly measured, but can be calculated as backspacing minus 1/2 of overall rim width. A negative result means negative offset.
Enthusiast discussions of wheel fitment most always go straight to the question of what overall size will fit their ride-16x8? 17x8.5? 18x9? The inevitable answers are: maybe, maybe, and maybe. Take two different 16x8 wheels out of the box and one may fit your car perfectly while the other won't even come close. The difference often comes down to two related terms: backspacing and offset. Backspacing is the position of the wheel mounting surface in relation to the back face of a rim, usually expressed in inches. Offset is the position of that mounting surface in relation to the centerline of rim width, and is confusingly enough normally stated in millimeters, positive or negative. A rim that has its mounting surface positioned outboard of its width centerline is said to have positive offset, so that an offset spec of +32, for instance, means the mounting surface is 32 millimeters outboard of the center of rim width. A mounting surface right on the rim centerline has zero offset, and one that is inboard has negative offset.
Backspacing/offset, then, determines how far inboard or outboard a rim of any given width is positioned, and this can make all the difference when trying to fit a wheel without contacting chassis bits or wheelwell lips. Wisely, many manufacturers now offer the same rim in a variety of offsets. In addition to simply avoiding clearance issues, choosing within an appropriate backspacing/offset range can also help position the rims so they properly fill the wheelwells for an aggressive stance. Remember that when measuring for clearance, that the overall width of any wheel is about an inch greater than its stated (nominal) width, because nominal width is measured across the inside (bead) surfaces of the inner and outer flanges. So a 16x7 wheel is actually closer to 8 inches in overall width.
You're Braking Up
As brake packages for classic Fords have grown in variety and size, so has the need for rims of commensurately large diameter, simply to clear the circumference of rotors and calipers. It should be fairly obvious that if 13-inch rotors and multi-piston calipers are in your plans, a 15-inch rim just isn't gonna cut it. But caliper overhang-the distance the face of the caliper extends laterally beyond the wheel mounting surface-is another critical factor of brake bulk that must be measured. You might find a wheel that would otherwise fit just fine, except for the fact that the back of its spokes won't clear your huge calipers.
This tight spoke/caliper clearance...
This tight spoke/caliper clearance highlights the importance of careful measurement of "caliper overhang"-the distance the caliper face protrudes outboard of the hub mounting surface. If your project will have serious calipers, disregarding this one measurement can ruin your day, even if you get all your other clearance measurements right, especially up front where the brakes are at their beefiest.
A potential time-saver is...
A potential time-saver is this multi-adjustable Wheelrite tire and wheel simulator from Percy's High Performance (www.percyshp.com). You mount it on your hub and use its adjustability to experiment with various rim diameters, widths, and backspacings. The wire can be set to approximate tire profile. Then (at normal ride height) you rotate the hub and see if there are any clearance issues-pretty handy for around $100.
Here we have some rims off...
Here we have some rims off of two of CDC's Dynacorn-based '67s. One is 15x7 with a P215/65R15 tire; the other has P245/40R18 rubber on an 18x8 wheel. Both are of roughly the same overall height and demonstrate the concept of plus sizing, but notice also that the wheel mounting pad is notably more outboard on the 18-inch combo, meaning its offset is more positive than that of the 15-incher. In this case, the offset difference required had to do with differing track widths and brake packages between the two vehicles.