Muscle Mustangs & Fast FordsHow To Wheels Tires
Fitting Larger Wheels and Tires on Your Mustang
Besides adding aggressive looks to your Mustang, a larger wheel and tire package can increase performance. But simply slapping on wider meats without careful consideration can result in tire interference and/or deteriorated handing. When rubber hits metal, rubber will always lose. Fender lips and suspension components will quickly slice a tire to shreds, which not only ruins the tire, but is extremely dangerous.
Each car is a little different, so taking your time and doing your homework will save you headaches down the road.
With your existing wheels and tires mounted and the car on level ground, see how much extra room you have on the outside and inside of the wheel well. Make sure you take into account suspension parts and check for clearance when the wheel goes up and down in the wheel well. On the front, check for clearance when the wheels are turned. You may need to install steering rack travel limiters to keep the tires from rubbing. You’ll need to give yourself a little safety factor when the tire sidewalls flex during cornering. Check both sides, and note the smallest of all your measurements.
Example: On this 1995 Mustang, the front tires were maxed out, but the rear tires (if we rolled the fender lips—more on this later) could be one-inch wider on the inside and one inch wider on the outside.
Specify a Tire
Using your current wheel and tire as a guide, you can now specify a wider package that will fit. With the wheel removed, measure its overall width. Add this measurement to the additional widths you noted previously. This is the maximum “section width” of tire that will fit. (Note that the section width is not the same as the “size” of the tire.)
Fortunately, sites such as Tire Rack list the section width for each tire when it’s mounted on its “measured rim width.”
Example: The 2003 Cobra wheel and its 275/40-17 tire have a section width of 10 1/2 inches. Since we figured the tires could be one-inch wider on the inside and one-inch wider on the outside, our target section width is 12 1/2 inches. Using Tire Rack’s website, we found a 315/35ZR17 tire with a 12.6-inch section width that was a good match.
Specify a Wheel Width
With the tire specified, we need to find a wheel that will fit the tire. A good general rule of thumb is to the “measured wheel width” for the tire you want to use. This ensures the tire’s contact patch and cornering response are maintained as designed.
Example: Our 315/35ZR17 tire would work well on a wheel that’s 11-inches wide.
Specify a Wheel Offset/Backspacing
This is probably the most important part of wheel and tire fitment. We need to specify a wheel that puts the tire where we want it. There are two ways to measure where a wheel fits in relation to the inner and outer fenders: “offset” and “backspacing.”
Offset is the distance between the center line of the wheel’s overall width and the mounting surface. Positive offset wheels have the mounting surface biased towards the outside of the car (most modern cars are this way).
Backspacing is the distance between the back edge of the wheel and the mounting surface. It’s more easily measured “in the field” so wheel manufacturers will often provide this measurement.
Example: The 2003 Cobra wheels on this 1995 Mustang GT have a 26mm offset and a 6-inch backspacing. Since our desired wheel and tire package “grows” the same one-inch on the inside and outside, the 26mm offset doesn’t change. However, the backspacing increases by one inch, to 7-inches.
If you’re getting custom wheels, simply supply the specifications to the manufacturer, and they’ll take it from there. However, if you’re buying “off the shelf” wheels, you’ll have to do some digging.
Some custom, multi-piece wheel manufacturers will send you a couple various options for test-fitting purposes. While this adds time and shipping cost, it’s good piece of mind if you’re unsure of your measurements. You can also check with other people with similar combinations. But, keep in mind: we’ve seen many wheel/tire combinations that owners claim “fit fine” are a few rubs away from failure.
Since most wheels only come in 1/2-inch width increments, sometimes a wheel spacer is needed to get the fitment “just right.” However, wheel spacers should only be used as a last resort, should be from a reputable source, and must be installed according to the manufacturer’s instructions. Wheel spacers may require longer wheel studs. Tightening the wheels to the manufacturer’s torque specifications (usually around 95 ft-lbs) is absolutely necessary when using wheel spacers.
1979-2004 Mustangs have a thick pinch weld seam on the rear fender, which is just waiting to slice your new tires to pieces. Buy, borrow, or rent a fender roller and follow the instructions to create the necessary clearance for your tires.
1. We used a plumb bob on the outside of the fender on this 1995 Mustang GT to help us determine that our wheel and tire combination could be one inch wider to the outside. (Note that the wheel lip will have to be “rolled” to gain the necessary clearance.)
2. We measured the clearance on the inside of the wheel well, and found the quad shock to be the area of least clearance. We found that a one-inch wider wheel tire and combination toward the inside was a safe amount. This, combined with our outside measurement, meant we could fit a 2-inch wider wheel and tire combination on the rear of this 1995 Mustang.
3. We measured the section width of our existing wheel and tire combination using a straight edge and a tape measure. The 275/40-17 tire on this 9-inch wide 2003 Cobra wheel measured 10 ½-inches.
4. Since we calculated our 1995 Mustang could accommodate a 2-inch wider wheel and tire combination, we needed a tire with about a 12 ½-inch wider section width. Using the Tire Rack’s website, we found a 315/35-17 tire with a 12.6-inch section width when fitted to an 11-inch wide tire.
5. Sometimes a wheel spacer is needed to get the fitment “just right.” However, wheel spacers should only be used as a last resort, should be from a reputable source, and must be installed according to the manufacturer’s instructions.
6. 1979-2004 Mustangs have thick fender lips that limit wheel and tire clearance. Buy, borrow, or rent a fender roller and follow the instructions to create the necessary clearance for your tires.