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Frequently Asked Questions

Calculated Tire Results?

When using our tire calculators to get tire size dimensions and revolutions per mile, the results are mathematic calculations based on if every tire was made to the exact same specifications. As you may know this is not the case. Tires from each manufacturer have small variances in diameter and width from one another. In most cases the calculated dimensions will be very close, but this is why we provide a list of tires that are manufactured in each size. After calculating, you can select individual tires to check the actual manufacturer specs for that tire. Keep in mind, tire specs are measured with the tire mounted on the measured rim width and not installed under the weight of a vehicle.

Most tires on our site will provide a 'Measured Rim' spec, this is the width of the rim that the tire was mounted on when taking the spec measurements. The tire width will change slightly when mounted on a different rim width. Generally the width will change about 2/10" for every 1/2" in rim width when mounted within the tire's rim range.

Actual revs per mile are physically measured with the tire under load. This makes the tire shorter and therefore spins more times than an unloaded tire and makes calculating the specific revs per mile difficult. As a general rule revs per mile will be 3% more than the unloaded calculation. For example if a tire is calculated to have 100 revolutions per mile under no load, then the tire will spin closer to 103 times per mile when under load.

Tire Size Visualizer?

Our tire size visualizer is built to display the correct size profile for the tire size input. No matter the diameter of the calculated tire size, the visual height of the tire images will remain the same. The wheel image and tire width image will adjust to give the visual appearance relative to itself, not another tire size. If we changed the size of the tire images relative to other sizes, then a 54" tire would be too big for the page and a 23" tire would be too small.

As for the tire comparison visualizer, depending on your tire size inputs the visualizer makes the bigger size the full height, and the smaller size relative to the bigger size. Again, this allows us to make full use of the page for displaying the images for you to see, and make them display the correct profile.

Alternate Tire Sizes?

When you make a tire calculation it will populate a list of alternate sizes with a tire size, Diameter, Width, and Wheel column. The diameter column is showing you the alternate tire size diameter in inches, along with the percentage difference from the calculated size. The width column is showing you the alternate tire size width in inches. The wheel column is showing you the wheel diameter of the alternate tire size along with a range of wheel widths that the alternate tire size is approved to be mounted on. In some cases you will see an (extra) text that is showing the rim range for the Light Truck version of the alternate size. This will only occur when there are both versions on the tire available, otherwise the one range is for all tires.

For instance, if it says +2.5% then that alternate size is 2.5% taller than the calculated tire size. Generally, it is recommended to not exceed a 3% difference when changing from the OEM size on a non-modified vehicle. A green row means that the alternate tire diameter matches the calculated tire diameter. However it does not mean that the tire will definitely work for your application, the width of the tire still needs to be accounted for.

Speedometer Error?

When changing your tire size from OEM you may experience some speedometer error. This is because your speedometer is reading revolutions, and is calibrated based on the factory tire diameter. When you change your tire diameter you are traveling a different distance per revolution than before. However, the results may be different than the calculations suggest.

Most of the time vehicle manufacturers error their speedometer to show a faster speed than you are actually going (up to 4%). Also tire wear and inflation pressure can error your speedometer on top of that. If the factory speedometer is 3% fast and your tires are half bald, you may already be 4% off. If you are assuming your speedometer is correct but it's actually 3% fast, when you install 3% taller tires your speedometer will now be reading correctly even though you installed larger tires.

WHAT? Then why have the speedometer calculator at all? Well, it is still correctly calculating the relative error between the two tire sizes. You just need to know if/how much your speedometer is already off.