We know most of you are going to supersize your rims when you upgrade. To avoid as many fitment hassles as you can, try to keep the overall outside diameter of your tire the same as the wheel grows inside it. That's a concept called "plus sizing." For each inch in diameter the wheel grows, the sidewall has to shrink a corresponding inch to keep the overall diameter the same. Why is that so important? There are a lot of computerized engine and driveline functions that are based around how far your tires travel when they make one revolution. When a tire guy talks about rpm, he means revolutions the tire makes per mile, not engine speed per minute. Changing the overall diameter will change the tire's rpm, which can throw off your speedometer and antilock brake system and also affect how quickly the car accelerates.
Going from a 15- to 16-inch rim is considered plus-1. From a 15- to a 17-inch rim is plus-2, and so on. Here's an example of how you can plus-size tires straight from a popular tire retailer's Web site. Say you're starting with a stock combo of 205/65R15 tire on a 15x7 rim. A plus-1 conversion would up the wheel size to 16x7.5, and the tire would change to a 225/55R16. A plus-2 conversion would mean going to a 17x8 wheel and a 235/45R17 tire.
Note that tire sidewall height isn't the only thing affected by plus-sizing. The tire's width grew, too. When sidewalls get shorter, the tire needs to get wider in order for it to maintain the same amount of air volume, and therefore maintain its load carrying capacity. If you don't keep the air chamber the same size, load capacity is reduced. That means the tire will have to work harder to hold up your car, which can generate excess heat, the leading cause of tire failure.
In some cases there's fudge room with a tire's load capacity. If you have a small car, or one that you've lightened for racing, you may be able to get away with using a tire that has a lighter load rating. But be very careful if you go down in load rating, and double-check your intent with a tire pro before you make the change.
There's a ton of information about your tires printed on the sidewall. Size, sure, but also speed rating, load-carrying capacity, treadwear, traction and temperature grades, recommended inflation pressure, and more.
Here's what the size numbers mean. For a 205/55R16, the 205 designates the tire's section width (sidewall-to-sidewall) in millimeters; 55 is its aspect ratio, or the ratio of its height to its width (the lower the number, the shorter the sidewall); and 16 is the wheel diameter that will fit inside the tire.
These days the tire's load index and speed ratings follow the size numbers. The load index is expressed by a two-digit number and the speed rating is indicated by a letter-we've included charts in this story to translate what those numbers and letters mean. Then there's the Uniform Tire Quality Grade, or UTQG. These are ratings, required by the U.S. Department of Transportation, of the tire's treadwear, traction, and temperature resistance.