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 wheels
This article is one in a series that have been released in conjunction with Wayne's
book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything
from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive
step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website
for more details.
Check out some other sample projects
from the book:
For this project, I polled a number of people on the internet mailing list, Rennlist.org, in an attempt to figure out what the best and most popular combinations of tire and wheel sizes were for the various models of 911. Unfortunately, I confirmed what is inherently true about almost all 911 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 911.
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. Early tires had inner tubes, much like common bicycle tires. It’s very uncommon to find these tires still fitted to 911s these days, however, if you pick up an older car, it may indeed still have the original tires on it.
With the advent of new materials, the tubeless tire has basically become the tire of choice. The tubeless tire has a bead that seals itself automatically against the rim when inflated. Tubeless tires are easier to mount on wheels, deflate slower when punctured, and can be temporarily repaired without removing the tire from the wheel rim.
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 width of about 127 mm. The letter following the width and length is the tire’s maximum speed safety ratings:
Q=99 MPH, 160km/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, it’s usually a bit more complicated, because most 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, most 911 owners do not drive their cars in the snow or the rain.
In an ideal setting, such as on the race track, 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 hydroplane 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 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 a 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% 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 911 that has been in storage 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? The early 911s didn’t have any fender flares on either the front or the rear of the car. As a result, it’s difficult to fit anything other than a six inch wide wheel under either the front or rear fenders without adjusting the fenders or the suspension. The very rare 911R seven inch wheel will fit, but at last check, these wheels were about $1000 each. 15” or 16” diameter wheels should fit fine, as long as they aren’t wider than six inches. In order to accommodate a 7” wheel to these early cars, you can trim the inside of the lip on the fender well, or you can roll the fenders outwards. Sometimes you can make a wider rim fit by installing a smaller profile tire, but this defeats the purpose of the larger rim, and may also make the tire fit poorly.
Rolling the fender involves using a baseball bat or long piece of pipe on the inside lip of your fender to push it out slightly. Many people do this in order to get a larger wheel or a larger tire to fit. However, many people don’t prefer the look of the rolled fender, and it could adversely affect the resale value of the car. If you give the car a bit of negative camber, you can also make the 7” rims fit in the rear.
The 1969-73 cars had a slight fender flare both front and rear, as did the 1974-77 cars. However, easily fitting a seven inch wheel under these fenders is difficult. To complicate the situation even more, there are many different tire sizes available nowadays that weren’t available back when the cars were new.
With many of the older cars, the tire sizes that you can fit on the car often depends 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.
A 205/50/16 tire will fit nicely on the six inch wheels that came stock with the early 911s. For a bit more aggressive profile, try installing a 225/50/16 on either the six inch rims or the seven inch rims. Unfortunately, tire sizes differ from each manufacturer, so one size from one company may fit better than another. This is another good reason to find a good tire shop who will tolerate fitting the tires specifically to your car. The factory owner’s manual from 1974 recommends 165HR15, 185/70VR15 or 215/60VR15 for the stock 911. Unfortunately, good performance tires for 15” wheels are getting very difficult to find. The options are much greater for cars equipped with the 16” wheels.
In 1978, Porsche widened the flares on the rear of the 911SC, giving much better options for installing tires. The factory also increased the width of the rear wheel to seven inches. The factory owner’s manual for the 911SC indicates that the standard tires for the 911SC are 215/60VR16 for the front and 225/50VR16 for the rear. If you can’t find the 215 sized tire, a good 205/55R16 will suffice.
With the wider wheels, the options for the installation of tires grow exponentially. As with the early cars, rolling the fender can help to accommodate a wider wheel and tire. The rear fenders on the 1978-89 body can be rolled out to fit an eight or even nine inch wheel. The fronts can sometimes fit a stock seven inch wheel combined with a 225/50/16 tire without modifications, but it depends on the car.
It’s also possible to place some larger 17” wheels on the 1978-89 chassis. 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.
So after reading this project are you still confused? You should be, and rightly so. It would appear that there is a never ending amount of options for tire sizing on the various 911 models. 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 (www.pelicanparts.com) or on the internet email list Rennlist.org. Bruce Anderson’s book, Porsche 911 Performance Handbook, also has a real handy table on recommended wheels and tire sizes for all year 911s. Regardless, you will find that everyone will have an opinion to share, and a wheel/tire option that they have tried on their car.
Figure 1
Shown here is a slightly rolled fender with a seven inch wheel under it. Under some circumstances, the tire and wheel will fit with an appropriate amount of clearance. Differences in tire sizes, and differences in the bodies and construction of the 911 may give different results for different combinations of wheels and tires. Have patience, do some research, and make sure that you take your car to a tire shop that will let you try out different sizes of tires.
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