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.
This is not the case as tires from each manufacturer have small variances in diameter and width from one another.
In most cases it 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 a specific rim width and under no load (not installed on 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 2/10" for every 1/2" in rim width. 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, therefore we add this 3% to our calculated revs per mile.
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.
When you make a tire calculation it will populate a list of alternate sizes with a tire size, equivalent and diameter column.
The equivalent column is the alternate tire size converted to inches, the height-X-width-R-wheel are shown.
The diameter column is showing you the alternate tire size diameter in inches, along with the percentage difference from the calculated size.
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.
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 install new tires that are 4% taller, use a GPS App to check your speed it may look like your speedometer is reading correctly.
WHAT? Then why have the speedometer calculator at all? Well, it is still correctly calculating the error based on the two tire sizes. You just need to know if/how much your speedometer is already off.