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.

For example, a 255/45R17 tire still has a design section width of 10.04" (255mm = 10.04 inches). But 85% of 10.04" is 8.534", which rounded to the nearest half inch, is 8.5". Ideally then, a 255/45R17 tire should be mounted on an 8½x17 rim. If your tire is narrower than the rim, according to Rim Ringz™ proper sizes (Wheel and tire sizes) - or  which is in instruction in every package box of Rim Ringz™,it can happened that Rim Ringz™ will NOT fit properly, and it could fall off the rim.

Example rim depth

The 18 X7JJ part of that is the size of the wheel rim - in this case it has a depth of 7 inches and a diameter of 18 inches. Those information you can find on each wheel (outside or inside wheel). On most aftermarket wheels, they don't want to pollute the lines and style of the outside of the wheel with stamped-on information - it's more likely to be found inside the rim, or on one of the inner mounting surfaces.

Tire too wide or too narrow

Given all the information above, you need to know one last thing. A rim that is too narrow in relation to the tire width will allow the tire to distort excessively sideways under fast cornering. On the other hand, unduly wide rims on an ordinary car tend to give rather a harsh ride because the sidewalls have not got enough curvature to make them flex over bumps and potholes. That's why there is a range of rim sizes for each tire size in Rim Ringz™ proper sizes (Wheel and tire sizes) - or  which is in instruction in every package box of Rim Ringz™. If you install a  225/40R18 tire on a rim narrower than 7.5 inches or wider than 7.5 inches, you might experience undesired results.

If your tire is narrower than the rim according proper sizes (Wheel and tire sizes) - or  which is in instruction in every package box of Rim Ringz™, Rim Ringz™ will not fit properly and then could fall off the rim. The tire size is the most important thing for the proper use of Rim Ringz™. Rim Ringz™ is designed according to tire and wheel standards.

Also, if your tire is wider than the recommended size for your particular rim, the tire could lose some air. Any deviation from the recommended size for your particular rim can have a negative impact on  Rim Ringz™ and it could fall off the rim. Even if at first it seems that  Rim Ringz™ will fit the rims, but the tire size does not match the wheel size, we recommend you do NOT install  Rim Ringz™.