Ford Mustang Wheel Fitment - One Size Does Not Fit All
Correct Wheel Fitment is Paramount for Safety, Looks, and Handling
November 01, 2006
By Mark Houlahan, Wayne Cook
Photography by Wayne Cook
One of the most important improvement we do to our classic Fords is upgrade the wheels and tires. Not only do wheel and tire sizes usually increase, but we can also take advantage of technology that is ever evolving.
Replacing a steel wheel with one made of aluminum reduces unsprung weight, and that's always a good thing. Another plus is that an aluminum wheel can be a thing of beauty, while a stamped-steel rim is rarely so.
The easiest way to measure...
The easiest way to measure backspace is to put the wheel face down on the ground. Lay a straight edge across the rim of the wheel. Using a ruler or tape measure, determine the distance from the straight edge to the face of the flange mounting pad. This measurement is the wheel backspace. On this American Racing rim, the backspacing is 331/44 inches. The offset is the distance from the face of the mounting flange to the true centerline of the wheel.
The wheelhouse depth must...
The wheelhouse depth must be carefully measured in order to determine what width wheel can be accommodated. Be sure to allow at least 11/42 inch on either side of the wheel for tire sidewall protrusion in your calculations. Take the measurement from the inner wheelhouse to the inside edge of the fender lip.
When it comes to tires, few would argue that a bias-ply or bias-belted tire is better than a modern radial. The only exception we can think of is for concours-show usage. Modern radial tire technology just keeps getting better and better, and this is something many classic Ford enthusiasts should take advantage of.
The first issue in any wheel and tire upgrade is determining whether the desired combination will fit your car. You'll want to fill the wheelhouse completely while having no part of the tire protruding from the car and no interference during steering or suspension travel. Remember also that once you mount a tire to a wheel, you've bought both. At that point, no company will refund your money if the combination you've selected doesn't fit your car.
We'll examine some popular wheel and tire upgrades, and talk about some of the different aspects of size that you need to be aware of before you shop for a new set of rolling stock. To begin, you'll need to determine the fitment parameters of your car. First, you must measure the backspacing on the wheel you want and the wheelhouse depth on the car you have.
Early in the decision process,...
Early in the decision process, look at the potential for brake-caliper interference if your car has disc brakes. Templates are available from brake manufacturers to help determine if your brakes will work with the wheels you want. They may also have a list of workable combinations, although it's difficult to keep track of every possible combination. Even with the templates and a manufacturer's list of applications, the best way to be certain a wheel will clear the brake calipers is to test the fit before you buy the wheels or mount any tires. If necessary, purchase one wheel for the test. Most wheel shops will refund your money if the wheel is returned undamaged and has never had a tire mounted. We test fitted this wheel at all four corners before any tires were mounted.
With the wheel seated on the...
With the wheel seated on the axle flange, check for adequate clearance on the inside edge of the rim. Remember that the car's body will tend to roll into the top of the tire's inner sidewall during turns.
Another clearance checkpoint...
Another clearance checkpoint is this rear-suspension snubber mounting. The outer edge may need to be folded inward or cut off entirely.
Another important factor in...
Another important factor in wheel and tire selection is the location of the rear leaf springs in relation to the surface of the axle flange. Once the depth is known, it can be compared to your backspacing to determine if there's enough clearance between the wheel and the spring.
This perspective of the installed...
This perspective of the installed wheel shows how close the wheel and tire will come to the spring. Our 17x9.5-inch wheel with its 5-inch backspacing is a cozy but workable fit. By the time we allow for tire sidewall bulge, there will be less than 1 inch clearance on the inside edge.
Even stepping up 1 inch to...
Even stepping up 1 inch to a 15-inch wheel will make a big difference in the way your car looks and handles. These American Racing Torq-Thrust D wheels along with P225/55R15 tires make a nice first upgrade, but even this modest size improvement should be checked for clearance in all dimensions.
When installing a larger wheel...
When installing a larger wheel and tire assembly, be sure to check for interference at the control arms, tie-rod ends, and shocks. This 16-inch Vintage 45 wheel will have no problem working in the wheelhouse of a '69 Mustang.
We recently measured for custom...
We recently measured for custom wheels on a project car at Classic Creations of Central Florida for the installation of a 17-inch wheel package. After checking our caliper clearance, backspacing, and wheel-house dimensions, we settled on our wheel and tire choice. The wheels are Wheel Vintiques Billet Bullet sized 17x9.5 in back and 17x8 in front. Our tire choice is a Nitto NT555 P275/40ZR17 sized for the rear axle and P245/45ZR17 for the front axle. With the tire mounted, the assembly tucks up into this '65 Mustang rear fenderwell quite nicely, though this axle has been narrowed and the rear leaf springs moved inboard for larger tires. In this tight rear tire installation, be sure to check the positioning of the parking-brake cables because they can rub completely through the tire inner sidewall in a mile or two, and you won't hear it happening-although you might smell burning rubber.
This installation of a 17-inch...
This installation of a 17-inch Cragar SS wheel on a '65 Mustang is a much tighter situation and near the limit on the early car. Be sure to check this arrangement for clearance problems by cycling the steering wheel lock to lock. On cars with power steering, it is especially important to check for interference at the power-steering hoses.
This Autoworks project car...
This Autoworks project car has an even larger 18-inch wheel on the rear axle. The company specializes in custom wheels and tires and their fitment to classic Fords. While the trend has been to go larger, the number of fitment hurdles grows with every increment of increase. Because this '67 Mustang convertible is in primer, it's hard to tell that the rear fenders have been carefully flared to accommodate the larger wheel and tire assembly.
On applications where you...
On applications where you have no choice on offset, the difference can sometimes be compensated for using a spacer set. That is how these late-model Mustang GT five-spoke wheels fit this early car. They would look great on almost any classic Ford.
On this 20-inch installation,...
On this 20-inch installation, clearances are closer still. As the body rolls during cornering, the inner sidewalls on the rear tires will rub if the wheelhouse is not massaged. This car is destined for the show circuit. You can use the following table to point you in the right direction for sizing. Fitting a wheel and tire package is different for each vehicle, but staying within these guidelines will enhance your chances for a good and workable fit. Use the dimensions of your existing wheel and tire package as a reference to determine the dimensions of the new wheel/tire package.
To fit the late-model Mustang...
To fit the late-model Mustang GT wheels onto the early car, a spacer kit like this one can be used. It is available from several sources and is a simple solution that works well.
Make sure a qualified professional...
Make sure a qualified professional mounts your tires. It's easy to both damage a tire and get hurt with a tire machine if you don't know exactly what you're doing. We had Pit Crew Tire Service's mobile tire center come out to mount our Nitto NT555 tires on the Classic Creations project car. Pit Crew's high-end Hunter TC3500 tire-mounting machine installs tires easily without touching the rim. Once the tire is seated, it will be inflated with nitrogen for minimal leakdown and optimum tire life.
Many wheels, both factory...
Many wheels, both factory and aftermarket, are not centered using the central wheel opening. Rather, the wheel is centered on the bolt pattern for optimum balance when installed onto the car. This is called being lug-centric. The problem is that many tire stores just use a cone and don't have the lug-plate fixtures shown here, which are used to center the wheel perfectly on the tire-balancing machine. If you're having trouble balancing your styled-steel wheels or Magnum 500s, check to see if your balance shop is using the correct centering fixture.
This is our project's Wheel...
This is our project's Wheel Vintiques wheel and Nitto tire mounted on the Hunter Road Force spin-balance machine inside Pit Crew's mobile tire truck. This state-of-the-art balancer even simulates the vehicle load using pressure from a roller. This replicates actual operating conditions for the most accurate balance possible. Notice the lug-plate in use on the wheel, too.
|'65-'66 MUSTANGS |
|Wheel ||Backspace ||Tire Size |
|15x6 ||4.000 ||P215/70R15 |
|15x7 ||4.250 ||P225/60R15 |
|15x8 ||4.500 ||P235/60R15 |
|16x7 ||4.250 ||P225/60R16 |
|16x8 ||4.500 ||P225/50R16 |
|17x7 ||4.250 ||P205/50R17 |
|17x8 ||4.500 ||P235/45R17 |
|'67-'70 MUSTANGS |
|Wheel ||Backspace ||Tire Size |
|15x6 ||4.000 ||P215/70R15 |
|15x7 ||4.250 ||P225/60R15 |
|15x8 ||4.500 ||P225/60R15 |
|15x9 ||5.500 ||P235/60R15('69-'70 only) |
|16x7 ||4.250 ||P205/55R16 |
|16x8 ||4.500 ||P255/50R16 |
|17x7 ||4.250 ||P205/50R17 |
|17x8 ||4.500 ||P235/45R17 |
|17x9 ||5.500 ||P245/40R17 (P275/40R17rear axle) |
|18x10 ||6.500 ||P225/40R18 |
|20x10 ||5.000/custom ||P225/35R20 (rear axle only) |
Why Nitrogen in Tires?
Craig Knarich of Pit Crew Tire Service explained to us that since nitrogen is denser than oxygen, the larger molecules escape less easily from tires, resulting in a more gradual loss of pressure over time. Research shows that a tire inflated with nitrogen loses its pressure three times slower than if it were inflated with air. As a rough rule of thumb, if your tire pressure is 20 percent below the optimum, you'll reduce tire life by up to 50 percent and the additional rolling resistance can add 10 percent or more to your fuel consumption. Across the life of a tire, that's a big financial penalty. The Department of Energy reports that 4,000,000 gallons of gasoline are unnecessarily wasted every day due to low tire pressure.
Nitrogen is also moisture-free, so pure nitrogen-inflated tires experience less steel-belt and rubber degradation, resulting in longer tire life. The use of nitrogen also reduces valve and wheel corrosion. Nitrogen-inflated tires also run cooler and require less maintenance, according to a Goodyear application bulletin. Non-flammable nitrogen technology has been used in race-car tires for over 30 years.
Decoding Tire Codes
The series of letters and numbers on a tire's sidewall tell a lot about what you're getting for your money as well as the tire's capabilities. The first series of numbers denotes tire size. After that, the two most important things to look for are the service description and the uniform tire-quality grade or UTQG.
Using the information in the service description, we know that a set of our P215/60R16-94T tires will carry a load of 5,908 pounds at up to 118 mph safely. While the load range is fine for our '67 Mustang, the 118 top speed could easily be exceeded, calling for a tire with a higher speed rating. We should note that the speed rating doesn't apply to tires that are worn out, repaired, damaged, retreaded, or not properly inflated.
|LOAD INDEX ||CAPACITY (IN LBS) |
|71 ||761 |
|72 ||783 |
|73 ||805 |
|74 ||827 |
|75 ||853 |
|76 ||882 |
|77 ||908 |
|78 ||937 |
|79 ||963 |
|80 ||992 |
|81 ||1,019 |
|82 ||1,047 |
|83 ||1,074 |
|84 ||1,102 |
|85 ||1,135 |
|86 ||1,168 |
|87 ||1,201 |
|88 ||1,235 |
|89 ||1,279 |
|90 ||1,323 |
|91 ||1,356 |
|92 ||1,389 |
|93 ||1,433 |
|94 ||1,477 |
|95 ||1,521 |
|96 ||1,565 |
|97 ||1,609 |
|98 ||1,653 |
|99 ||1,709 |
|100 ||1,764 |
|101 ||1,819 |
|102 ||1,874 |
|103 ||1,923 |
|104 ||1,984 |
|105 ||2,039 |
|106 ||2,094 |
|107 ||2,149 |
|108 ||2,205 |
|109 ||2,271 |
|110 ||2,337 |
While the wheel and tire assembly...
While the wheel and tire assembly turn on the balancer, information regarding the state of balance is reported on the computer screen using Hunter's SmartWeight system. The machine then instructs the operator where to place the weights and describes how heavy they should be in order to correct the imbalance completely.
With most aftermarket wheels,...
With most aftermarket wheels, there is no provision for balance weight attachment as there is on factory wheels. The solution is these self-adhesive lead weights that can be applied to the wheel without damaging the polished or chromed surface. They are available in exacting increments down to 11/44 ounce, or 7 grams. The higher-quality weights shown in grams are much more accurate than the "chocolate bar" weight at the top of the photo. Craig Knarich of Pit Crew Tire also stressed the use of the proper lug nut for the wheel being used. He said aftermarket wheels often come with the wrong lug nuts and can cause vibration issues.
Let's look at a tire sized...
Let's look at a tire sized at P215/60R16-94T. This is an average tire size you might consider for a '67 Mustang. The first number, 215, is the tread width in millimeters. The next number, 60, is called the aspect ratio. Here, sidewall height is expressed as a percentage of tread width. In this case, it's 60 percent, or slightly more than half the width of the tread. The R16 tells you that it's a radial tire made to fit a wheel 16 inches in diameter. Understanding the tire size code will enable you to purchase exactly the right sized tire for your needs.
The second half of the service description is known as the speed rating. Devised to give the buyer an awareness of the tire capability for safe operating speed, it is the T in the 94T service description. Three of the most common speed ratings are S, H, and V. The complete speed-rating table is as follows.
|SPEED RATING ||MAX MPH |
|M ||81 |
|N ||87 |
|P ||93 |
|Q ||99 |
|R ||106 |
|S ||112 |
|T ||118 |
|U ||124 |
|H ||130 |
|V (VR) ||149 |
|W (ZR) ||168 |
|Y(ZR) ||186 |
The federal government requires...
The federal government requires tire manufacturers to grade tires in three areas. Known as the Uniform Tire Quality Grading System, it provides guidelines for making relative comparisons when purchasing new tires or evaluating tires put on new vehicles. This is helpful, but it is not a guarantee of how a tire will perform or how long it will last because many factors have a significant influence on tire performance and longevity. First, the tread wear grade gives an indication of a tire's relative wear rate. The higher the tread wear number, the longer it should take for the tread to wear down. For example, a tire grade of 400 should wear twice as long as a tire grade of 200. Second is the traction grade, which is an indication of a tire's ability to stop on wet pavement. A higher grade means a shorter stopping distance. Traction is graded from highest to lowest as "AA", "A", "B", and "C". Third are the temperature grades, which are an indication of a tire's resistance to sustained heat From highest to lowes
While the first series of...
While the first series of numbers describe the tire size exactly, the last portion of the code, which is 94T, is known as the service description. It is divided into two parts, the load index and the speed rating. The load index for cars and light trucks ranges from 70 to 110. The load index table is shown below. Capacities listed are for a tire that is properly inflated.
An alignment should be considered...
An alignment should be considered mandatory on any new wheel and tire purchase. We have seen an improperly aligned car completely ruin a set of new tires within 500 miles. Many tire shops include an alignment with a new tire purchase, so shop around.