Tire and Wheel Sizing
on Your BMW
Wayne R. Dempsey

Difficulty Level: 4
Difficulty scale: Adding air to your tires is level one
Rebuilding a BMW Motor is level ten

[click to enlarge]

     For this project, I polled a number of people on Internet chat boards to figure out the best and most popular combinations of tire and wheel sizes for the various models of the BMW 3-Series. I confirmed what I believed to be inherently true about almost all hardcore BMW 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 BMW.

     Let’s talk for a few moments about tires in general. Although you can write volumes on tire sizing and design, we’ll try to cover only 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 width with respect to its height. An example of a common European tire size is 195/65R15. The first 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 127 mm. The “R” following the width and length denotes radial construction. The next number is the tire’s inner diameter, in this case 15 inches. Finally, the number followed by a letter is the load rating, and the letter is the tire’s maximum speed safety ratings. Speed ratings are as follows:

 
     There is an exception to all this: If a tire is Z-rated, the “Z” is noted immediately after the height and before the “radial” denotation. If our example above were Z-rated, it would be 195/65ZR15. A good Z-rated tire should be more than adequate for non-suicidal driving!  Tread is another important consideration in selecting a tire. You should select your tire based on what type of driving you plan to do. With the BMW 3-Series, it can be a bit more complicated, because some people don’t drive them in all types of weather. For example, an all-weather tire seems like a natural choice for a family sedan located in a snowy environment. However, many BMW 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 cause the car to hydroplane by elevating the wheel onto a wedge of water.

     The full array of choices for tire tread is way beyond the scope of this project. One rule of thumb, though, is to make sure you purchase a tire appropriate for your climate. Using a snow tire or an all-weather tire on a BMW that is rarely driven in the snow will significantly reduce the tire’s contact patch area and also reduce cornering performance on dry roads. However, failing to equip your car for bad weather can result in disastrous effects if an unforeseen storm catches you unprepared. Of course, if you drive your car only during the dry summer months, look for a conventional performance tire with a maximum contact patch area.

     Another important consideration is tread wear versus traction. Tread wear refers to the average number of miles that the tires can handle before they 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 vary according to each driver’s personal driving habits, but the various ratings are good for comparisons among different brands and types of tires.  Traction relates to the type of materials used in the tire. The harder the rubber used in the tire, the longer the tires will last. However, hard rubber provides much less traction. An “A” rating for traction is best. These tires will grip the road well, but will generally wear out faster than the “B” or “C” tires.

     It is also important to consider longevity when selecting a tire. Most tires have a limited 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. This is also an important consideration if you are purchasing a BMW that has been in storage or sparsely driven for many years. Although the tires may have plenty of tread on them, they actually may be dried out and ready to fail. If tires develop cracks in their 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, which tires and wheels can you fit on your BMW 3-Series?It all depends upon the wheel design, offset, and type of tires you prefer to run on your car. With so many different combinations out there, it’s impossible to document them fully in a mere few pages. I did create a wheel collage (opposite page) you can use for generating ideas about which wheels to mount on your car. This array contains photos of BMWs 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. When going this route, I recommend the use of BMW-specific hub-centric spacers, which are located on the hub by a machined center hole, as opposed to lug-centric spacers located by the position of the lug nuts alone.

     Rolling the fender involves using a special tool to fold in the inside of your fenders so you can fit wider tires (see Photo 2). Many people perform this modification in order to get larger wheels or tires to fit. However, there are those who don’t like the look of rolled fenders, so this could adversely affect the resale value of the car. If you give the suspension a bit of negative camber (angling the top of the wheel inwards), you can also make the larger rims fit on your car.

     With many of the older cars, tire sizes that fit often depend upon the condition of the car. Sometimes the chassis is perfectly balanced from left to right, and sometimes it is 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 to find the best fit. Go in during 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, 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.

     If you want to go with larger wheels on the E36, the best combination for 18-inch wheels would probably be 225/40R18 in the front and 245/35/R18 in the rear. For more tire patch on the rear, you can probably fit a 255/35R18, but the best handling comes from having the same sized tires and wheels on all four corners. If you are willing to roll the fenders on your car and run spacers, the number of wheel options increases for you. With wider wheels, tire options grow exponentially. Again, rolling the fender can help accommodate a wider wheel and tire. 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 you know which types of wheel and offset you have before you attempt to mount tires.

     So after reading this project, are you still a little bit confused? You should be, and rightly so. It would appear that there is a never-ending set of options for tire sizing on the various BMW models. The best way to figure out what type of tires to place on your car is to ask around. Check on the Internet—and check into the chat room of one or more technical bulletin boards—like the one at PelicanParts.com. I’m also fond of the TireRack.com website; it has useful tools for determining the right wheel/tire combinations to fit your car. Regardless of what the “pros” think, you will find that the majority of BMW owners have an opinion to share and a wheel/tire option they have tried on their car.

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