We all know that wheels come in many different diameters – anywhere from the tiny 14” steels to 24” chrome monsters and beyond. But wheels also come in different widths, and the width of the wheel not only affects how the wheel sits on the car but also how the tire fits on the wheel. Here's a look at how wheel widths affect you.
Wheel sizes are defined as (diameter x width), so that a 17” diameter wheel might be 17x7”, 17x7.5” or 17x8”. Widths tend to scale up with the diameter, so that while you will almost never see a 17x5” or a 17x10” wheel, a 14x5” or 19x10” wheel are standard sizes.
While it's pretty easy to determine your wheel diameter, (The last number of your tire size will be the diameter, ex. 235/45/17 means the tire fits a 17” wheel) it's not as easy to determine the width. On most wheels the width will be printed on the back of the spokes, meaning that the wheel has to be removed to read it. If the width is not printed on the back, you may have to measure. Take a tape measure and measure from the inside of each flange, that is, from the spots where the tire and wheel make contact, rather than from the outer edges of the wheel.
Many high-performance rear-wheel drive cars, especially BMW and Mercedes sedans, have what are called a “staggered” setup, meaning that the rear wheels are an inch wider than the fronts. This provides for a wider wheel and tire, and therefore a larger contact patch on the rear drive wheels. This is a wonderful thing, but it requires some attention to detail by the owner. For one thing, it means that the wheels cannot be rotated from back to front, since while the front wheels will fit just fine on the rear, putting the rear wheels on the front will not fit properly and will probably cause the tires to rub against the suspension. In addition, the front and rear tires will probably be two different sizes, meaning that care must be taken when buying and mounting tires to ensure that the sizes are correct and that the correct tires go on in the correct positions.
As with wheels, tires also come in many different widths. Certain tire widths are approved for the corresponding wheel width, meaning that the tire is wide enough to properly fit the wheel. Unfortunately, it is often possible to fit a narrow tire onto a wheel that is too wide for a proper fit by forcing the sidewalls to spread wider than they are designed for. It's pretty easy to recognize this problem, as it leaves the tire sidewalls leaning at a diagonal to the tread instead of vertical. This is extremely bad. Tire sidewalls are supposed to be vertical, since they're what's holding the tire tread stable against the car's weight and protecting the wheel against impacts.
Even more unfortunately many people, in my experience mainly tuners and teenagers, have come to see this useless and dangerous condition as being an acceptable “look”, as if having tires that look “stretched” somehow cancels out the physical idiocy of having tires with sidewalls at a 45-degree angle to the wheel. I literally have this conversation at least once a month:
“You know that your tires are too narrow for your wheels, right?”
“It's the 'stretched' look.”
“Yeah, well the 'stretched' look is the reason your tires are shredded and your wheels have that 'beat to crap' look.”
I'm all for expressing one's individuality on a car, but unless you're rocking those new tires with printable sidewalls, your tires are not worth screwing with just to get a “look.” Any responsible tire installer will have a book that lists the approved tire sizes for a certain rim width. Any responsible tire installer will refuse to mount tires that are too narrow for the wheels. The key word there is “responsible.”