Section Smarts
The only way to know exactly the section-width dimension is to measure it with the tire mounted on the wheel as shown. Tire manufacturers publish section-width information based on a given wheel width.
When someone asks, "How wide is that tire?" the universal answer is always with regard to tread width. But the savvy tire stuffer knows section width is the real key to measuring tire width. Section width is the distance across the tire at its widest portion of the sidewall with the tire mounted on a given-width wheel. The section width will change slightly depending on the width of the wheel. The tire companies publish dimensions of each size tire based on a given wheel width.
For example, a BFGoodrich 275/40ZR17 g-Force T/A KD tire has a section width of 10.9 inches when mounted on a 9.5-inch-wide wheel. BFG also states that for every 1/2 inch of wheel-width change, the section width will vary by roughly 0.2 inch. So if we were to mount that same tire on a 9-inch wheel, the section width would be reduced to 10.7 inches. There is also an ideal wheel-width range for any size tire. BFG recommends that this size tire be used on wheels ranging from 9.0 to 11.0 inches. On an 11-inch wheel, the section width would be a wider 11.5 inches.
Don't assume that all same-size tires have identical section widths. Tire diameter is closely maintained, but section width has a tendency to vary between manufacturers. We have measured the section width of the same size BFG, Firestone, and Goodyear tires mounted on exactly the same wheel and discovered major differences in section width. This means you can't assume that just because your buddy's BFG tires fit cleanly that your Goodyears will generate the same clearance.
Measure Twice, Fit Once
This simple, homemade tool works best on the rear and can be used to simulate either the wheel width or the section width of the tire to establish proper clearance. The tool can be used on the front, but it's more difficult to ensure that everything will clear. The problem is simulating ride height while being able to turn the wheel to check for clearance. Try placing the floor jack directly underneath the lower ball joint and raising the jack to ride height. This is especially important because the front suspension moves in an arc inboard as it moves up into the wheelwell.
In the performance world, the idea is to stuff as much tire and wheel under the wheelwells as possible. Many car crafters make the mistake of getting all amped up over buying a trick set of wheels and tires only to discover the big meats don't fit. A better plan is to measure twice or even three times and then go with what you know will fit. The easiest and ideal way to measure is to just bolt on a set of wheels and tires from a buddy's car to see if they will fit your car. That's not always the most convenient idea, but it's ultimately the best.
If you are going to measure your clearance area, there are several variables that come into play that affect tire clearance. The most important one is ride height. The worst thing to do is to measure the car for tire clearance, purchase the tires and wheels, and then decide to lower the car. If you are considering dropping the stance, do it before you purchase the tires and wheels. A lower ride height will affect especially the front tire clearance and may be the difference in one whole tire-size change. For example, at factory ride height, an early Camaro might be able to accommodate a 255/50R16 tire on a 16x8-inch wheel, but lower it by 1 inch, and that size might drop to a 245/50R15 tire. The inner wheelwell clearance is the issue; it becomes much tighter with a lower ride height.
To fit the widest tire-and-wheel package, the car will need the maximum wheel backspacing on the frontend. Stock upper control arms may contact the wheel, or sometimes (depending on wheel diameter) the tie-rod end will limit the amount of backspacing. The next thing that will often hit on full-lock turns is the sway bar. A minor amount of sidewall rub at full lock could be acceptable since the only time this would occur is in parking situations.
Here, this front tire-and-wheel package is too far inboard, contacting the sway bar long before the steering hits the stops. Always check clearance at ride height with the steering at full lock.
We've built a homemade wheel backspacing and sidewall section-width checker out of a piece of aluminum and a length of all-thread. We drilled two holes to mount the gauge to the brake hub and then drilled a series of holes from the axle centerline out to simulate the radius of a 15-, 16-, and 17-inch wheel. Then we drilled three more holes to simulate the radius of tire diameters of 26, 27, and 28 inches. The length of all-thread is placed through either a wheel or tire hole (or both with two lengths of all-thread) and then inside or outside dimensions can be checked.
We also found a professional tire-and-wheel fitment tool that is far more sophisticated and will quickly determine wheel diameter, width, backspacing, and tire profile. This tool is more expensive than our homemade gizmo, but perhaps you can find a shop that has one of these tools to help you find your data much more quickly. Percy's offers a tool called the WheelRite simulator for $89.99 through its Web site.