Today, everybody's got custom wheels. The guy driving the Escalade to work runs 24s with paper-thin sidewalls and those ridiculous spinner wheels. Even the pizza-delivery kid's got 18s on his Honda. You also see these same poseurs stranded along the freeway or stuck in a parking lot with a squared-off wheel or sliced sidewall for their efforts. So how do you make your mark on the boulevard big spin?
Careful measurement and double-checking your work will be rewarded with a good-looking wheel-and-tire package that doesn't gouge your tires or your wallet.
Just like camshafts and the super-size-me option at your local burger stand, everybody defaults to "bigger is badder," and nothing abuses that statement more than fat tires and wheels. We'll look at both the cool, corner-turning stuff and drag-race meats with this story, where the goal is to stuff big tires under the wheelwells. This is a challenge but rewarding as hell when it works. The key here is to do your research first, then buy. Too many mistakes are made when the urge to spend the big bucks on a set of fat tires and wheels is just too great before you really know what will fit. We'll look at those short-sidewall hero tires and tell you how to fit big, sticky rear tires under your supercar. Just remember, once those tires are mounted on the wheels, you can't take 'em back.
Gimme My Backspace
While big, fat tires may be the ultimate point of this exercise, we need to start with wheels. Most car crafters are familiar with wheel diameter and width. These two dimensions are easy to measure. But in the art of stuffing fat tires, it's the backspace that makes all the difference. It's just not cool to jack up a car to fit fat rollers. That's a disaster for so many reasons we shouldn't have to warn you about them. The aggressive, mean-spirited corner burner is all about a low stance and big meats on all four corners. The simple solution is to stuff the wheel-and-tire package as far inboard as possible while clearing everything that conspires to get in your way.
The way to do this is with wheel backspacing. This is the measurement between the wheel mounting flange and the inner wheel lip. Some manufacturers still refer to backspacing as offset, but this is confusing. It's much simpler to merely refer to the amount of backspacing. For example, let's say we have a 17x8-inch wheel with 4 inches of backspacing. This places the mounting flange in the middle of the wheel width. Today's wheel manufacturers have figured out that even the older musclecars offer room to fit wider tires and wheels if the wheels come with more backspacing. Today, it's not unusual to see a 17x8-inch wheel with 5 to as much as 7 inches of backspacing. As backspacing distance increases, the mounting flange moves closer to the outside edge of the wheel. This is called a positive offset. For example, all C4 and later Corvette wheels are built this way.
The inside diameter of the wheel is also important if your car is equipped with larger-than-stock disc brakes. Most brake companies are less than specific about minimum wheel diameter clearance, mainly because each wheel company's drop dimension for a given diameter varies with the style of the wheel. This means you can't assume, for example, that all 16-inch wheels will clear a set of Wilwood 13-inch rotor brakes. The only way to know for sure is to mount a wheel on your hub and measure the caliper clearance.
Measuring wheel width is easy. This is the inside distance between the wheel lips. | 
Measuring backspacing is almost as simple. Lay a straightedge across the wheel inside the wheel lips and measure from the mounting flange to the straight edge. Make sure the horizontal tool does not include tire sidewall. Some wheel companies measure the lip as part of the backspacing dimension while others don't. The difference is only about 1/4 inch, but it's worth knowing, since that might be the difference you need. | 
Another important point that could cause problems is caliper overhang, which is the amount of clearance between the outboard portion of the disc-brake caliper and the inner portion of the wheel face. Vintage Wheel Works' illustration shows this very clearly (arrow). Wheel drop also affects caliper clearance. |