Mustang Wheel and Tire Performance Upgrades

When it comes to overall vehicle performance potential the tire is the most important single component. The tires, with help from the wheels and suspension, are responsible for putting the power to the ground, cornering, stopping, ride quality, and even less obvious things such as fuel economy and interior noise.

 

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In this chapter I cover the most popular wheel and tire options for a first-generation Mustang. I focus on performance, comfort, wear, life, and cost. I can’t cover every possible combination so I provide useful guidelines and recommendations for the three vehicle types (daily driver, performance street car, and streetable track-day car). I also discuss tire pressure monitors.

 

Wheel Sizes

In general, wheels tend to be more of an aesthetic choice than a consideration for performance. But a wheel and tire upgrade is necessary to extract the maximum performance from the vehicle

There are 14-, 15-, or 16-inch reproduction-style wheels to fit your car that are easy to find. Most are steel reproductions, although there are some aluminum wheels in these sizes as well.

 

 

 

 

 

 

17-Inch Wheels

Wheels larger than 17 inches offer a larger contact patch for increased performance but this may create fitment and clearance issues with the bodywork. A 17-inch wheel has enough sidewall height to provide good ride comfort while the lower sidewall (versus a 16-inch or smaller wheel) provides improved steering response and feel. You should still be able to use a 45 or 50 aspect ratio tire with a 17-inch rim.

Single-piece aluminum or billet wheels can provide excellent strength and design flexibility (for plenty of style options) at very reasonable prices. While they aren’t as light as two- and three-piece wheels they are stronger and resist damage from potholes or flexing under extreme cornering. Special casting technologies are used with two- and three-piece designs, some using a steel outer ring for extra strength.

 

 

Vintage Wheel Works offers wheels for early Mustangs. They carry classic styles and have a variety of finishes and other appearance options (center caps, color combination, etc.). Forged wheels in 17-inch size offer exceptional strength for their weight but they are typically $500 per wheel or more. They are offered in one-, two- or three-piece versions and have a limited selection in available styles. Those offering minimal weight may also not be as well suited to dealing with the impacts encountered in street use.

In general, unless you’ll be seeing quite a bit of track time (and have the suspension modifications to make best use of it) the case for going all out with a relatively more costly forging is usually not all that compelling at the 17-inch wheel size.

 

 

18-Inch Wheels

An 18-inch wheel provides maximum track-day performance and some degree of streetability. This rim gives you the lowest practical sidewall height without making the wheel too weak or too heavy. The early Mustangs, particularly 1965/1966 cars, are better suited to this wheel because they typically clear the suspension and allow the use of a wider wheel with more favorable backspacing. A 17-inch rim also does this to some extent but the 18-inch wheel provides more clearance for a larger brake caliper.

Cast, composite, and forged wheels are available. Castings tend to be on the heavy side and the composites can be less suitable for the extreme and uneven impact forces from potholes, etc.

Care still must be taken, however, when making the final choice because some 18-inch forgings are rather fragile. A simple design, such as basic fivespoke, split five-spoke, mesh-pattern, etc., typically offers the best balance of weight and strength.

 

 

 

20-Inch and Larger Wheels

Wheel diameters of more than 18 inches are best thought of as being used primarily for their looks, not for performance. Although some (such as the 20-inch Weld RT-S) can be suitable for performance and track use they still suffer from certain compromises relative to an 18-inch counterpart. When comparing similar 18- and 20-inch wheels the latter is heavier, more expensive, inherently less strong unless extra material is added to compensate, less able to tolerate impacts such as potholes (due to the lower sidewall tire it must use), and provides a harsher ride.

Other factors such as tire wear and noise are often also negatively affected. Other than appearance, about the only thing that can be considered to be an improvement in performance is steering response, due mostly to the lower sidewall tire. If a heavier wheel (such as a casting) is chosen to reduce cost the polar moment of inertia (PMOI) likely increases significantly, thus requiring a brake upgrade (more cost and weight, etc.) just to maintain equal stopping power.

From an aesthetic perspective, there’s also the potential for the brakes to get “lost” in the much larger wheel due to the increased gap between the rotor/drum and the inside of the rim. A larger rotor (bigger drums are hard to come by) is pretty much a given to avoid having such an unsightly gap. The bottom line is that unless you’ve done some very extensive suspension and braking modifications to your early Mustang it’s very unlikely you’d see any performance gain by going over 18 inches in diameter.

You will, however, have more cost, more weight, and several other negatives to contend with including the fact that most of these larger wheels are not exactly “classic” in their styling. With few exceptions (such as the Weld RT-S) fitment options are also limited unless you’re willing to cut the bodywork—seems like a lot of trouble just for a certain look with less-than-maximum performance.

When you change to a much larger, wider wheel, fit is a critical factor. Backspacing becomes much more important and brake caliper clearance is a prime consideration. Likewise, when fitting larger brakes you need to be sure the calipers clear the spokes as well as the inside of the rim.

Weld Racing offers high-end forged wheels in the RT-S series for early Mustangs. These forged wheels offer an excellent balance of weight and strength plus Weld also makes an extra effort to ensure proper strength for street use with little, if any, weight gain. Furthermore, Weld also prioritizes the reduction of PMOI, often referred to as the “flywheel effect.”

 

 

PMOI has a greater impact on performance than does weight alone. By carefully using finite element analysis plus other design and manufacturing techniques, Weld is able to put material where it does the most good for strength without increasing PMOI. This provides improved acceleration and braking, especially under transient conditions.

Perhaps the best feature of the Weld RT-S wheels is that they are offered in a wide range of offsets, widths (4 to 18 inches), and diameters (15, 17, 18, and 20 inches) plus three different pad heights, which provide clearance for even the largest aftermarket brakes. This wide variety and fitment flexibility is possible because Weld makes the RT-S wheels by precisely welding together two forged rim shells (which can easily be varied in size) with a forged center (which can have different styling, sizing, and pad heights) into a very strong, yet relatively light, assembly. They fit virtually any combination of brake and suspension parts you may put on your early Mustang without the need for doing costly bodywork to avoid rubs.

If you decide to go over 18 inches you need to seriously consider an upgrade to larger brakes as well.

 

Wheel Fitment Guidelines

Regardless of what size wheel you decide to use there are certain guidelines you can follow to help ensure there aren’t any fitment problems. These come from shops such as Mustangs Plus and wheel manufacturers such as Weld Racing that have decades of experience selling and installing various components on stock and modified first-generation Mustangs. They often include information in their catalog and on their website to help customers make decisions and are, of course, also available by phone and email to answer more specific questions. Also be sure to ask around and look on Internet forums to find what’s worked for others with a setup similar to what you have.

Here are a few general guidelines for fitment considerations:

Backspacing

First you need to decide if you’re going to make the wheels fit the car or modify the car to fit the look of the wheels. Do you absolutely need a very deep-dish look on the wheels? Do you want to have different and/ or staggered wheel sizes front to rear? If so, you may have to tub the car, narrow the rear axle, flare the fenders, etc. These procedures are neither inexpensive nor easy. If you are flexible with the look of the wheel, you can likely find a backspacing to fit.

For fitment decisions, it’s better to use backspacing than offset because the former is an actual measurement whereas the latter is a calculation based on the theoretical centerline of the wheel (it doesn’t account for manufacturing tolerances).

You measure backspacing by laying a straightedge across the rear edge of the wheel/rim and then measuring straight down from the lower edge of the straightedge to the hub/ flange surface of the wheel (where it contacts the rotor or drum). Be sure to keep the ruler or measuring tape perfectly vertical because any tilting or angularity will introduce some inaccuracy.

Extreme backspacing may provide the look you want but it also greatly increases the loading on parts of the wheel and on the wheel bearings. Independent suspensions are more sensitive to extreme backspacings than is a solid axle. Extremes can also increase the “scrub radius” on the front wheels, thus further negatively affecting handling and braking performance. It’s best to have relatively neutral backspacing (i.e., the wheel center is close to the center of the rim).

Drum/Rotor Thickness

If you are converting drum brakes to disc brakes (or even from single-piece rotors to two-piece/composite rotors) make sure you consider the fact that the drums and rotors can have different thicknesses and thus affect the fi nal/mounted location of the wheel. Discs are normally thicker than drums, and two-piece rotors (which use an aluminum center/hat) are often thicker than one-piece iron rotors. These push the wheel toward the fender unless any difference has otherwise been compensated for. It’s best to mock up the wheel on the car and measure the clearances before mounting and balancing the tires so you can change the backspacing if you need to.

The size and shape of the brakes also matters in terms of internal clearance to the wheel. Radial clearance (to the rim) and lateral clearance (to the spokes) have to be addressed. Multiple-spoke profi les provide greater fi tment fl exibility but not all wheel manufacturers offer them. Many provide a worksheet for you to take specifi c measurements to send to them so they can verify if their wheel will fi t before you buy it. Many brake manufacturers provide templates that represent the shape of their calipers to mock up inside the wheel to help determine if you have adequate clearance.

Rim Width

You can use a slightly wider rim if you minimize lateral movement of the suspension. In the front this is mainly a function of the suspension geometry and can be affected by moving the control arm mounting points and/or using stiffer bushings (or bearings) instead of soft-rubber OEM parts. In the rear you’re basically talking about the leaf-spring bushings and shackles.

Switching to polyurethane or, preferably, Delrin (such as Global West’s Del-A-Lum) greatly reduces lateral movement of the axle and thus allows you to use a slightly wider rim and tire because the axle stays put. A coil-spring suspension with a panhard rod is not as effective in this regard because it causes the axle to travel trough an arc, thus actually introducing lateral movement. For coil-spring suspensions a Watt’s linkage is the preferred option because it does not introduce lateral movement.

Additional rim width can also be accommodated by “massaging” the fender lip with Eastwood’s Fender Rolling Tool for the inner side of the tire. There’s usually not much you can do in the front other than make sure the brake hoses are secured out of the way. In the rear you can generally gain clearance by bending the forward edge of the inner wheel housing because this is generally where the tire hits first.

Make sure you stay within the tire manufacturer’s recommended width range when choosing your wheel. Wider is not always better but it is always heavier (for the same wheel design). You want to ensure the tire shape is correct when the vehicle load is on it. Too wide or too narrow a rim distorts the tire, reduces performance, increases wear, and, potentially, is less safe. While this is a bit less critical with radial tires it is very important with bias-ply tires. If information from the manufacturer is not available you can use the resting/static bead width of the tire to get a good approximation of the optimal wheel width.

 

 

Lugnuts

Whatever wheels you decide on you need lugnuts to mount them to the hub. With stock wheels the standard OEM-style lugnuts usually suffice. When you put down some serious money for new rims and tires, however, you want to do what you can to make sure they don’t get stolen. Locking lugnuts are easy to find and not that expensive but some are better than others.

Instead of using one large locking lug it’s better to go with a locking lug on each stud. This makes theft much less likely, they look better, and they don’t create an imbalance. These lugs require a special key or socket to remove or install them. The lugs themselves have some form of external pattern that the key/socket matches so a normal socket doesn’t work. The best ones are also tapered so locking pliers, for example, slide off.

 

 

For race cars you may also need special lugnuts that allow the wheel studs to protrude through them to be legal for some forms of racing. They can also be lighter in weight to provide a small reduction in PMOI. Unless you’re racing, they should probably be avoided because they’re generally not protected from the elements so they rust and corrode easily.

 

Tire Categories

Tires are the “rubber” that meets the road. As such, they are responsible for transferring all of the forces generated by the vehicle to the road. Tires are generally divided into categories to indicate their capabilities. The following list is for passenger cars only, in the order of increasing dry-traction performance potential under “summer” conditions:

• Grand Touring
• High Performance
• Ultra-High Performance
• Max Performance
• Extreme Performance
• Track and Competition DOT

Clearly, I can’t cover every category for each of the three types of vehicles so I’ve made some assumptions most likely to apply to each of the vehicle types. Keep in mind, tires frequently get updated or are superceded by a newer product line so specific suggestions may no longer be available. However, because tire manufacturers generally have only one product per category you should be able to figure out what replaces the particular tire I use as an example.

First, I need to cover a few basics that apply to every tire: the tire size designation, service description, and the Uniform Tire Quality Grading (UTQG) ratings.

 

Tire Size Designation

Tire sizes are represented by four components; 275/40R18, for example. The “275” refers to the crosssectional width of the tire (not the tread width), the “40” refers to the aspect ratio (the cross section’s height divided by its width), “R” is for radial tires (versus bias ply), and “18” is the wheel diameter for the tire.

Performance tires have relatively wider section widths for better dry traction and lower aspect ratios for improved steering response and handling. The tradeoffs for these generally include a greater tendency to hydroplane (lose traction on a wet surface) and a harsher ride.

 

Tire Service Description

The service description is a combination of the load rating and the speed rating and is usually expressed with two or three digits and a letter; 88W, for example.

The digits refer to the load rating of the tire, which ranges from 70 to 110 with higher numbers being able to handle a higher load. You must ensure that the combined load rating of your tires exceeds what’s needed for the vehicle. This is almost always not an issue with a Mustang but it does become an issue with trucks when towing something and/or carrying heavy loads.

The letter in the service description refers to the speed rating/limit of the tire. You can go to a source such as tirerack.com to get the full listing of what each letter represents. For our purposes, the most relevant codes are:

• S = 112 mph
• T = 116 mph
• U = 124 mph
• H = 130 mph
• V = 149 mph
• Z = in excess of 149 mph
• W = 168 mph
• Y = 186 mph

If a Y-rated tire, for instance, has been tested to have a rating “in excess of 186 mph,” the service description is surrounded by parentheses such as “(88Y).”

 

Uniform Tire Quality Grading

The UTQG is generated from a federally mandated series of standardized tests and must be shown on the tire sidewall, as must the tire size and service description.

The UTQG is composed of three parts: a treadwear rating, a wet traction rating, and a temperature rating. An example is 300 A A; the “300” is treadwear, the first “A” is wet traction, and the second “A” is temperature.

Treadwear Rating

The treadwear rating is derived from actual on-road testing and is compared to the wear experienced on a standard test tire. If the treadwear of the test tire was the same as that of the test tire it would score 100. If it lasted twice as long it would get a 200 rating, and so on.

Because there is some room for interpretation in how each tire manufacturer calculates this rating, it is most useful for comparing tires within the same brand. Small differences between brands may not really tell you much but a larger difference can still be used reliably between brands to determine which tire will last longer.

Wet Traction Rating

The wet traction rating is meant to approximate the coefficient of friction of the tire while braking on a wet surface. It really is related more to the compound of rubber used in the tread than anything else. The ratings range from the highest “AA,” to “A,” then “B,” to a low of “C,” which signifies the worst wet traction.

Temperature Rating

The temperature test is used to find the maximum speed a tire can be run before heat buildup leads to its failure. The ratings are: “A” (more than 115 mph), “B” (100 to 115 mph), and “C” (85 to 100 mph).

All new tires sold in the United States must be able to get at least a “C” rating. Anyone looking at running the car on a track (or in a highspeed on-road event such as the Silver State Classic) should definitely not use any tire with a rating less than “A.”

 

Written by Frank Bohanan and Posted with Permission of CarTechBooks

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