It is important to ensure your vehicle is installed with a properly sized tire. Each tire size is designed to fit a specific rim width. The correct rim width allows the tire sidewall to flex where the designers meant for the flex to occur. There is a wide range of tires that will fit a specific rim width. After you know what size range of tire will fit your wheels, you can pick a size based on aspect ratio and height of the different tire sizes.
A rim that is too narrow in relation to the tire width will allow the tire to distort excessively sideways under fast cornering. As most of you know, tire stretching is not a good idea. If you're not familiar with the procedure, it involves stretching a tire beyond the recommended rim width range. For example, mounting a tire with a rim width range between 6" and 8" on a 9" wide wheel. The tire barely sits on the bead seat and creates a gap between the sidewall and rim.
Stretching the tire can lead to belt separation, sidewall splitting, shoulder separation, tire overheating, loss of air and poor tread wear- all of which place you and your car in harm's way.
On the other hand, unduly wide rims tend to give rather a harsh ride because the sidewalls have not got enough curvature to make them flex over bumps and potholes.
The following diagram illustrates the three scenarios.
There are two ways to determine the proper size wheel/tire fit. You can calculate it yourself or use an online fitment tool.
For 50-series tires and above, the rim width is 70% of the tire's section width, rounded off to the nearest 0.5. For example, a 255/50R16 tire has a design section width of 10.04" (255mm = 10.04 inches). 70% of 10.04" is 7.028", which rounded to the nearest half inch, is 7". Ideally then, a 255/50R16 tires should be mounted on a 7x16 rim. For 45-series tires and below, the rim width is 85% of the tire's section width, rounded off to the nearest 0.5.
Here are the calculation steps:
1. Measure the width of the rim. The measurement is taken just inside the lip or flange of the wheel that holds the tire bead. Wheel width measurements are taken in inches and you should be accurate to the nearest quarter of an inch.
2. Decide what type of tire in relation to the aspect ratio will go on the wheel. The aspect ratio is the ratio of the height of the tire to the width. If a tire size is 235/60R16, the aspect ratio is 60. Passenger car tires have ratios of 60 or 65 and sports car tires have lower aspect ratios of 30 to 50.
3. Divide the rim width by a factor to obtain the proper tire section width for the wheel. For tires with aspect ratios of 50 to 80 the factor is 0.70. For tires with aspect ratios less than 50 the factor is 0.85. For example, if your rim width is 7.5 inches and you want a tire with a 70 aspect ratio, divide 7.5 by 0.70 giving an answer of 10.7 inches.
4. Convert the result in inches to millimeters, multiply by 25.4 and round down to the nearest odd multiple of five. For the example, 10.7 times 25.4 equals 271. This would be rounded to 265, the nearest odd multiple of 5. Aspect ratios only come as odd multiples of five, such as 195, 205 or 215.
5. Look for tires with the section width and aspect ratio you calculated on the diameter of your wheel. If the example wheel is 16 inches in diameter, the desired tire size is 265/70R16.
To check on your specific tire/wheel combination, see the following link:
Whenever you are looking to install wider tires, try to get the diameter as close as you can to the one on your original tires. Here is an example of a 275/40-20 tire that was installed on a 2009 Challenger SRT that came stock with 245/45-20 tires and 9-inch forged Alcoa aluminum wheels. By going from a 45 aspect ratio to a lower 40 aspect ratio, the diameter of the tire was almost identical with only a -.07% deviation (see chart, below). This insured the accuracy of the speedometer and maintained the suspension level. In this case, the 9-inch wide width was the minimum width for this size tire for it to wear and perform properly.
Tire Size Tire Diameter (in.) Min./Max. Wheel Width (in.) Deviation %
245/45-20 28.68 7.5-9 N/A
275/40-20 28.66 9-11 -.07
305/35-20 28.41 10-12 -.96
275/40-20 tire on 9" Alcoa forged wheel
Cooper Zeon RS3-G1 tire mounted on 2009 Challenger SRT