Marc Christ
Brand Manager, Modified Mustangs & Fords
February 1, 2013

For most vehicles, a wheel and tire upgrade is categorized as an appearance modification. People put wheels and tires on their cars (and trucks) for a variety of reasons, and Mustangs are no exception. The difference is that Mustangs, unlike many other passenger cars, have a seemingly unlimited aftermarket that provides endless combinations to help you and your Stang achieve whatever it is that you want to achieve—be it street performance, open track capabilities, drag racing, vanity, or any combination of the preceding list.

There are numerous advantages to aftermarket wheels and tires over stock. But just like upgrading your suspension or engine, there are sacrifices that must be made if you want to improve handling or performance capabilities. Ford Motor Company uses strict guidelines when selecting or engineering wheels and tires for its automobiles. Safety, ride quality, fuel mileage, weather, road conditions, tread wear, noise, and many other factors are considerations when a manufacturer selects a tire for a certain car. Even performance vehicles, like the Shelby GT500, end up with a tire that has compromises.

So where do you start? Well, there are a lot of options for wheels and tires, and there are a lot of misconceptions as well. Numerous choices, mixed with confusing misinformation can cause people to make a knee-jerk reaction, by either buying the cheapest product, the product that "looks" the best, or even the product that other people are buying.

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There are advantages to following trends, and websites such as www.americanmuscle.com, www.latemodelrestoration.com, and www.tirerack.com can guide you make a purchase that you know will fit your vehicle. You can get free shipping, free mounting and balancing, and most brands and sizes are in stock. You can also rely on Internet forums to provide information, which can also be very helpful. But, if you desire, you can make your wheel and tire purchase a more interactive experience. And, in turn, make your Mustang (or other fast Ford) stand out from the crowd and perform better both on the street and on track.

The following pages will walk you through the essentials of wheels and tires, allowing you to make a more educated decision when selecting wheels and/or tires for your vehicle. This is geared more toward performance and handling, specifically for Mustangs, but is also applicable to other vehicles.

Wheels
Often incorrectly referred to as rims, a wheel is the spinning component that attaches to the axle. The rim is actually one part of a wheel, the outer part containing the lip and bead for the tire. The other two parts of the wheel are the hub and the spokes. These three components work together to direct driver input to the ground through the tires. They are merely a go-between in the quest for handling, and their design can either adversely affect or favor the overall performance of the wheel, tire, and suspension combination.

For the most part, the dimensions of a wheel are determined by the dimensions of the desired tires, the dimensions of the vehicle's wheel openings, and the size and location of other suspension components (like brake calipers). But other than size, there are other factors that play a role in a wheel's anatomy. Things like composition, backspace, and offset.

Most of us understand how wheel sizes work: a 17x10-inch wheel is a 17-inch-diameter wheel with a 10-inch-wide width. That's easy to see with a simple measuring tape. But the other measurements aren't quite so simple. Offset is the distance from the wheel's hub mounting surface (flange) to the centerline of the wheel. It can be either zero, positive, or negative. Positive offset is where the hub mounting surface is farther toward the outside edge of the wheel, and negative offset is where the mounting surface is more toward the inside edge of the wheel. It is usually measured in millimeters.

Backspace, on the other hand, is the distance between the mounting face and the back inside edge of the wheel. This dictates how far in the wheel will protrude into the wheel opening from the hub. This is very important when selecting a wheel, as you don't want the lip of the wheel or the tire to scrub the inner fender or framerail. Backspace is typically measured in inches, but can also be found in millimeters.

The composition of a wheel can also play a huge role in its performance and durability. The two major types of wheels are cast and forged. Cast wheels, like most factory and affordable aftermarket wheels, are poured into a casting mold to create the wheel shape. These are very durable, and relatively inexpensive to manufacture.

Forged wheels are reserved for premium aftermarket and racing wheels. This denser, stronger product can be much lighter than its cast cousin. Less rotating weight allows for quicker acceleration and better braking. There is a cost, though, when choosing a forged wheel for your Stang. While many cast wheels can be found for a couple hundred dollars each, a set of forged wheels starts at over $2,000.

Other measurements to consider when selecting a wheel are lug pattern and hub bore. Since these don't typically change with vehicle modifications, we wont really go into that.

Tire Specs
If you look closely, tires are riddled with writing, numbers, and symbols. But what does all that gibberish mean and why is it important? Some of it is inconsequential to you, the consumer, but much of it is valuable information.

Other than the brand name and line name (almost like a make and model of a car), the other prominent indication on the sidewall of all tires is the size. This is usually the number one factor when considering tires, so it's important to understand how the metric size designation works. A 245/45-17 tire has a section width (sidewall to sidewall) of 245 mm. This is not tread width, but actual width from the outside edge to the outside edge of the sidewalls.

The "/45" (in the 245/45-17 tire) refers to the sidewall aspect ratio. In other words, the distance from the wheel's rim to the tread (the sidewall width) is 45 percent the width of the section width. And the -17 (or R17 or ZR17) is the wheel diameter. The "R" designates that the tire is a radial, and "ZR" designates that it is a Z-rated radial, the only speed rating shown in tire size designations.

So to compute tire size using the English measuring system, you divide 245mm by 25.4 (the conversion factor), you get the section width in inches (9.65). Since the sidewall is 45 percent of that, you multiply 9.65 x 0.45, which equals 4.34 inches. And since we know the diameter of the wheel to be 17 inches, you get 17 + 4.34 + 4.34 = 25.68 inches diameter.This is how tall the tire is.

Immediately following the tire size is the service description, which consists of the load index and speed rating. The load index is a two- or three-digit number, ranging from 71 to 110, and indicates how much weight the tire is capable of bearing. This is not typically an issue when dealing with passenger car tires, especially Mustangs.

The speed rating, though, is very important. If you are going to be competing in drag racing or road racing events, be sure to have tires capable of the speeds that you will be achieving. Most production coupes and sport sedans are equipped with H (130 mph) or V (149 mph) speed-rated tires.

Z-rated tires are designated by the "ZR" in the size designation and were originally meant to be rated for speeds in excess of 149 mph. However, higher-rated tires have since been developed, due to the greater speed capabilities of newer production cars. Speed rating W (168 mph) and Y (186 mph) tires are typically meant for exotics, but you can purchase these tires as well. They usually also retain the "ZR" in the size designation.

Contact Patch
Though tires appear to be a very large part of the vehicle, the actual amount of tread contacting the pavement at any given time is very small. In fact, most tires have a contact patch no bigger than your hand. And the weight of the vehicle must be distributed across four of those small "contact" patches. Many factors contribute to a tire's contact patch, including tire width, aspect ratio, tire pressure, weight of the vehicle, and wheel width.

The few factors that are typically altered to apply a larger contact patch on a tire are tire width and tire pressure. After seeing how little of a tire is actually contacting the pavement, you can see how even adding 10 mm to the width of your rear tires will help you get much more traction on acceleration.

In drag racing, it's common for racers to lower their rear tire pressures to 16 or 18 psi (with drag radials), and slicks can be run as low as 10 psi or below. This allows the contact patch to be enlarged greatly, and the reduced pressure allows the wheel to "wrap up" the side wall to absorb the intial shock of the launch, thus aiding traction from a dig. Drag cars run skinnies in the front to lower rolling resistance by creating a much smaller contact patch. With a street car or open track car you aren't as concerned with creating a dead hook, but are more concerned with overall performance, be it acceleration, braking, or turning.

The Life of Your Tires
There are a number of factors that help preserve the life of your tires. Tire composition, driving style, alignment, tire pressure, and tire rotation all play a role in how long your tires last. But if you're good to your tires, and you don't put that many miles on them, your tires still may need to be replaced because of age.

Maintaining your tires to get the most out of them is easy if you do a few simple things. First, maintain proper tire pressure. Check your tire pressures every few weeks, or when you check your engine oil. A low tire won't only wear prematurely, but it's also dangerous—most blowouts are caused by incorrect tire pressure and can cause poor miles per gallon.

Another important aspect to the life of your tires is proper alignment. Immediately after installing a new set of tires, get an alignment. Replace worn suspension parts while you're at it. Your tires will thank you. And just because you got an alignment "a few months ago" doesn't mean that you don't need another one. Depending on driving habits and road conditions, you should get an alignment every six months to a year.

Finally, don't forget to rotate your tires. And just because you have directional tires or staggered wheels doesn't mean you can't rotate your tires. See the diagram on tire rotation for more information.

And if you've been good to your tires, and they've lasted a long time, you may have to replace them before they wear out. In fact, most tire manufacturers recommend replacing your tires after six years, whether they're worn out or not. But how do you tell how old your tires are?

Aging tires is easy. If you look on the inside of the tire, near the bead, there is a DOT number. At the end of that number is a four number date stamp. The first two numbers indicated the week of the year, and the second two numbers indicate the last two numbers in the calendar year that the tire was manufactured. So, for instance, if a tire was made in the eleventh week of 2013, then the date stamp would read 1113.

Selecting the Right Tires
With the help of tire distributors like www.tirerack.com, www.americanmuscle.com, and Latemodel Restoration, finding the right size wheel and tire combination for your car is quite simple. But if you take the time to do your research, use the tools we've provided in this article, and use a little imagination, you can set your Steed apart from the rest.

If you're going to drive your car mostly on the street, then buy tires meant for the street. If you track your car a lot, get a set specifically for that. As you can see, not all tires are built alike. Decide how much money you want to spend, start pricing out different combinations, and you may be surprised what you come up with.

Numerous choices, mixed with confusing misinformation can cause people to just make a knee-jerk reaction.

Many factors contribute to a tire's contact patch, including tire width, aspect ratio, tire pressure, weight of the vehicle, and wheel width.