Calculate tire diameter from a size like 225/65R17, or compare two sizes for speed, height, and mileage changes.
Table of contents
How to use our Tire Diameter Calculator
- Choose One tire to calculate one size, or Compare two tires to check a tire swap.
- Enter the tire code exactly like the sidewall format, such as 225/65R17; the middle number is a percent, not inches.
- In compare mode, enter Original tire size, New tire size, and Speed shown on dash.
- Use Advanced options if you want to change the Close-match limit or test a different Odometer distance to check.
- Sanity-check the results: a larger new tire should show a positive Diameter change, higher Actual speed at the dash speed, and fewer Revolutions per mile.
Definitions
Tire size: The sidewall code, such as 225/65R17, that gives tire width, aspect ratio, construction letter, and rim diameter.
Section width: The first number in a metric tire size, measured in millimeters. It is nominal tire width, not guaranteed tread width.
Aspect ratio: The number after the slash. It is the sidewall height as a percent of the section width.
Rim diameter: The wheel diameter after R, ZR, D, or B, measured in inches.
Sidewall height: The height of one tire sidewall from the rim edge to the tread.
Overall tire diameter: The calculated outside height of the tire from tread to tread.
Rolling circumference: The distance around the tire based on the calculated diameter.
Revolutions per mile: The estimated number of tire turns needed to travel one mile.
Diameter change: In compare mode, the signed percent difference between the new tire diameter and the original tire diameter.
Common mistakes and quick fixes
Mistake: Entering an inch flotation size like 33x12.50R15 in Tire size .
Fix: Use a metric tire code such as 285/75R17, because this calculator reads metric tire codes.
Mistake: Treating the middle number in Tire size as inches.
Fix: Read it as a percent of tire width; in 225/65R17, 65 means 65 percent.
Mistake: Swapping Original tire size and New tire size in compare mode.
Fix: Put the factory or current size in Original tire size so Diameter change has the correct sign.
Mistake: Leaving Speed shown on dash blank or using a negative speed.
Fix: Enter 0 mph or higher so Actual speed at the dash speed can be calculated.
Mistake: Using 0 or a blank value for Close-match limit .
Fix: Enter a positive percent, such as 3, so Diameter match status can compare the tire swap to your chosen limit.
Mistake: Reading Ride-height change as the full diameter difference.
Fix: Remember ride height changes by half the diameter change because only the tire radius raises or lowers the vehicle.
Limitations & Key Assumptions / Boundary Conditions
- This calculator uses metric tire codes such as 225/65R17. It does not convert inch flotation sizes such as 33x12.50R15.
- Results use the printed size code. Real measured tire diameter can differ by tire model, load, rim width, tread depth, and inflation pressure.
- Speedometer and odometer effects assume the vehicle was calibrated to the original tire size and that both tires roll according to the calculated diameter.
- The close-match status is only a planning check. It does not confirm wheel-well clearance, rim width, brake clearance, load rating, speed rating, suspension fit, or legal compliance.
- Rolling circumference is calculated from a simple circular tire model. Loaded rolling circumference on the road can be slightly different.
- Unusual but parseable widths outside the normal passenger or light-truck range may still calculate, but the result should be checked against tire maker data before use.
Methodology
How the tire code is read
The calculator reads a metric tire code by finding the section width in millimeters, the aspect ratio after the slash, and the rim diameter in inches after R, ZR, D, or B. Prefixes such as P, LT, T, and ST are accepted for entry, but they are not used in the diameter math.
width_mm = first width number
aspect_pct = number after slash
rim_in = number after R, ZR, D, or B
One tire calculations
Sidewall height is the tire width times the aspect ratio percent. Overall diameter adds the rim diameter plus one sidewall above the rim and one sidewall below it.
sidewall_mm = width_mm * aspect_pct / 100
diameter_mm = rim_in * 25.4 + 2 * sidewall_mm
diameter_in = diameter_mm / 25.4
Circumference uses the circle constant pi. Revolutions per mile divide the number of inches in one mile by the calculated circumference.
circumference_in = diameter_in * pi
circumference_mm = diameter_mm * pi
revs_per_mile = 63360 / circumference_in
Compare mode calculations
Compare mode uses the original tire as the baseline. Signed results are kept, so a positive diameter change means the new tire is larger and a negative diameter change means it is smaller.
diameter_change_pct = (new_diameter_mm - original_diameter_mm) / original_diameter_mm * 100
actual_speed_mph = shown_speed_mph * new_diameter_mm / original_diameter_mm
ride_height_change_in = (new_diameter_in - original_diameter_in) / 2
true_distance_miles = odometer_distance_miles * new_diameter_mm / original_diameter_mm
The diameter match status compares the absolute value of the diameter change with the close-match limit you enter.
if abs(diameter_change_pct) <= match_limit_pct, status = close match
Mini-example
For 225/65R17, the sidewall height is 225 * 65 / 100 = 146.25 mm. The diameter is 17 * 25.4 + 2 * 146.25 = 724.3 mm, which is 28.52 in. The circumference is about 89.58 in, so the tire makes about 707 revolutions per mile.