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Tire and Wheel Sizing on the Porsche 911 Carrera
 
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Pelican Technical Article:

Tire and Wheel Sizing on the Porsche 911 Carrera

Time:

4 hours4 hrs

Tab:

$300 to $1,000

Talent:

**

Tools:

Soft socket

Applicable Models:

 
Porsche 996 Carrera models (1999-05)
Porsche 996 Turbo, GT2, GT3 (2001-05)
Porsche 997 Carrera models (2005-08)
Porsche 997 Turbo, GT2, GT3 (2007-08)

Parts Required:

New tires, valve stems

Hot Tip:

Find a tire shop that will allow you to try fitting certain sizes and types of tires on your car, or find a friend who has a tire/wheel combination you like and borrow them.

Performance Gain:

Good tires can increase your handling and braking significantly

Complementary Modification:

Upgrade to larger or lighter wheels

For this project, I polled a number of people on a few Internet chat boards in an attempt to figure out what the best and most popular combinations of tire and wheel sizes were for the 911 Carrera. I confirmed what is inherently true about almost all hardcore Porsche owners--they love to modify and tweak their cars. Out of all the responses, no two were exactly alike. I've compiled and summarized the feedback here so that you can make an educated decision when equipping your ride.

Let's talk for a few moments on tires in general. Although you can write volumes on tire sizing and design, we'll try to cover the basics here. Tires are sized using a system that takes into effect the tire's aspect ratio. This aspect ratio is a function of the tire's height with respect to its width. An example of a common European tire size is 195/65R15. The number 195 refers to the width of the tire in millimeters. The second number, 65, refers to the height of the tire as a percentage of the width. Therefore 65 percent of 195 would give a tire height of about 127mm. The letter following the width and length is the tire's maximum speed safety ratings:

R=106 MPH, 170km/h

V=149 MPH, 240km/h

S=112 MPH, 180km/h

W=168 MPH, 270km/h

T=118 MPH, 190km/h

Y=186 MPH, 300km/h

U=124 MPH, 200km/h

Z=149 MPH, 240km/h and over

H=130 MPH, 210km/h

Needless to say, a good Z-rated tire should be more than adequate for non-suicidal driving! The last number in the tire size is the wheel diameter in inches. In this case, "15" refers to a 15-inch-diameter wheel.

Tread is another important consideration in selecting a tire. You should select your tire based upon what type of driving you are planning on doing. With the 911, sometimes it's a bit more complicated, because some people don't drive them in all types of weather. With a family sedan located in a snowy environment, an all-weather tire is a natural choice. However, many Porsche owners do not drive their cars in the snow or the rain.

In an ideal setting, such as on the racetrack, flat-surfaced tires called racing slicks are best because a maximum amount of tire rubber is laid down on the road surface. However, slicks have almost no traction in wet weather. The water has a tendency to get underneath the tire and help hydroplane the car by elevating the wheel onto a wedge of water as it is moving forward.

The array of choices for tire tread is way beyond the scope of this project. One rule of thumb is to make sure that you purchase a tire that is appropriate for your climate. Using a snow tire or all-weather tire on a 911 Carrera that is rarely driven in the snow will significantly reduce the tires contact patch area and reduce cornering performance on dry roads. However, not equipping your car for bad weather can result in disastrous effects if you are ill-equipped during an unforeseen storm. If you drive your car only during the dry summer months, then look for a conventional performance tire with a maximum contact patch area.

Another important consideration is the tread wear and traction. The tread wear refers to the average number of miles that can be put on the tires before they will need to be replaced. A tread wear indicator of 100 means that the tires should last about 30,000 miles. An indicator mark of 80 means that the tires will last 20 percent less, or 24,000 miles. Wear will be different for each car and each driver's personal driving habits, but the various ratings are good for comparisons among different brands and different types of tires. Traction is related to the type of materials used in the tire. The more hard rubber is used in the tire, the longer the tires will last. However, the hard rubber provides much less traction. A rating of "A" for traction is best. These tires will grip the road well, but will generally wear out faster than the "B" or "C" traction rated tires.

It is important to consider another factor in addition to tread wear when selecting a tire. Most tires have a shelf life based on the rubber's natural process of breaking down and becoming brittle. It doesn't pay to purchase a 30,000-mile tire if you are only going to be putting 3,000 miles a year on your car. After ten years, the rubber may be cracked and deteriorated beyond safe use, even if there is plenty of tread left on the wheel. This is also an important consideration if you are purchasing a car that has been in storage or sparsely driven for many years. Although the tires may have plenty of tread on them, they may actually be dried out and ready to fail. If the tires develop cracks in the sidewalls from aging, they can blow out when heated up from driving. A blowout is a very bad situation and can cause you to lose control of your car very quickly.

So, what tires and wheels can you fit on your 911? It all depends upon the wheel design and offset and which tires you prefer to run on your car. With so many different combinations out there, it's impossible to fully document them in a mere few pages. I did create a wheel collage (opposite page) that you can use for ideas on which wheels to mount on your car. This array was assembled from photos of Boxsters and 996/997 Carreras I took at various meets and club events over a period of three years.

Expanding the pool of options, you can also use spacers to accommodate different wheels that weren't originally designed for your car. I recommend the use of hub-centric spacers, which are located on the hub by a machined center hole. This is opposed to the lug-centric spacers that are located by the position of the lug nuts alone (see Figure 6 of Pelican Technical Article: Big Brake Upgrade for the Porsche 911 Carrera).

With some older cars, the tire sizes that you can fit on the car may depend upon the condition of the car. Sometimes the chassis are perfectly balanced from left to right, and sometimes they are slightly off from being in an accident or simply from body sag. It's best to find a tire shop that will allow you to try out several tires on your car in order to find the best fit. Go in the afternoon on a slow day and talk with your tire salesman to see if he will let you size the tires on your car. If he won't, then go to a different shop--there are plenty of them out there willing to cater to you, especially if you are going to shell out some money for high-performance tires.

Different tires will look and perform differently with various wheels, so the best option is to consult with a good tire shop for their recommendations. You can fit 20-inch wheels to a 911 Carrera, but at that point, the tire thickness is so small that the ride and handling suffers quite a bit. I don't recommend installing anything greater than 19-inch wheels. I currently run Genuine Porsche 18-inch Sport Design wheels manufactured for Porsche by BBS on the project 911 Carrera for this book.

With the wider wheels, the options for the installation of tires grow exponentially. The type of offset used on the wheel and the tire size will affect whether it will fit or not. The offset of a wheel is the distance of the center of the wheel from the edge of the mounting flange on the hub. Different wheels with varying offsets will affect tire sizing considerably, so make sure that you know which types of wheels and offsets you have before you attempt to mount tires to them. It's also important to keep in mind that Porsche made very similar looking wheels for the Boxster and the 996 Carrera, with the only major difference being the offset of the wheel. If you're buying used wheels, be sure that you purchase ones with the correct offset for the Carrera and not the Boxster.

So after reading this project, are you still confused? You should be--and rightly so. The Carreras came from Porsche with everything from a 7J x 17 on the front to the huge 12J x 19 on the mighty GT3. It would appear that there is a never-ending amount of options for tire sizing. The best way to figure out what type of tires to place on your car is to inquire around. Check on the internet at the various technical bulletin boards, like the one at PelicanParts.com. I'm also fond of the TireRack.com website--they have useful tools there for determining the right wheel/tire combinations that will fit on your car. Porsche also runs tests on aftermarket tires and puts out a recommendation for the Carreras. They offer this free from the dealerships, and when I asked for one I received over 50 pages of Porsche approved rims and tires. Regardless, you will find that everyone will have an opinion to share, and a wheel/tire option that they have tried on their car.

No matter which wheels you buy, you're going to want to make sure that you protect your investment with a set of wheel locks.
Figure 1

No matter which wheels you buy, you're going to want to make sure that you protect your investment with a set of wheel locks. The factory ones available from Porsche look pretty terrible though, and I don't necessarily recommend them. Booth Designs makes an aftermarket set of locks and polished studs that are of extremely high quality and really look like they should be on your Porsche (available from PelicanParts.com).

This photo shows how the aspect ratio of the tire changes with the change in wheel size.
Figure 2

This photo shows how the aspect ratio of the tire changes with the change in wheel size. In general, you do not want the outer circumference of the wheel to change--this will affect handling and also change your speedometer readings. So, when you go with a larger wheel, you must go with a lower-profile tire. The lower-profile tire results in less of an air pocket under the car, and this typically reduces the ride comfort. This photo shows some examples of three rear-wheel options for the 911 Carrera. It is generally my recommendation to run 18-inch wheels for the best combination of looks, performance, and ride.

The upgraded, polished lug nuts really look great on the 911 Carrera.
Figure 3

The upgraded, polished lug nuts really look great on the 911 Carrera. If you use them, be sure to use what is known as a "soft socket" (also available from PelicanParts.com). This special socket has a softer inner liner that will allow you to install your lugs without damaging them. If your center caps are looking aged, then you can try removing them and sanding them with some fine-grit sandpaper. The outer plastic covering on the crests gets old and cloudy after many years, and a few minutes of sanding can improve the overall appearance tremendously.

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