Free tool
Tire size calculator and comparison
A metric tire size gives the width in millimetres, the sidewall height as a percentage of that width, and the rim diameter in inches, which is enough to work out the tire's diameter. Enter one or two sizes to see diameter, sidewall, circumference, revolutions per kilometre and per mile, and how a new size changes the speedometer.
Metric sizes only: width / aspect ratio R rim, such as 225/70R19.5. Load index and speed rating are ignored.
Tire dimensions
| 225/70R19.5 | |
|---|---|
| Overall diameter | 810.3 mm (31.9 in) |
| Section width | 225 mm (8.86 in) |
| Sidewall height | 157.5 mm (6.2 in) |
| Circumference | 2,546 mm (100.2 in) |
| Revolutions per km | 392.8 |
| Revolutions per mile | 632.2 |
Sidewall = width × aspect ratio ÷ 100. Diameter = rim × 25.4 mm + 2 × sidewall. Circumference = π × diameter. Nominal sizes from the marking: a real tire varies by maker, tread depth, load and pressure.
Section width and rim diameter as defined in 49 CFR 571.109 and 571.110 (ecfr.gov, checked November 5, 2026). 1 inch = 25.4 mm, 1 mile = 1.609344 km.
Why compare tire sizes
A different tire size changes how far the vehicle travels with each turn of the wheel. A larger diameter makes the speedometer and odometer read low, a smaller one makes them read high, and both affect distance records, fuel figures and maintenance intervals. Comparing sizes before you switch shows how big the change is, and the vehicle manufacturer's approved sizes tell you which ones are allowed.
Distance from GPS, not from the tires
High Point GPS fleet tracking records every trip and the distance run from GPS position, with engine hours and fuel use from engine data for each vehicle. Maintenance reminders by distance or engine hours keep tire rotations and inspections on schedule, and FleetZap tracks the work orders for each truck.
Get a quoteTips before changing tire size
Use only sizes the vehicle manufacturer approves, listed on the tire and loading label or in the owner's manual.
Keep the same size on both sides of an axle and on every tire of a dual set, so the tires turn at the same rate.
Check the load index and load range of the new size: a tire with the same diameter can carry less weight.
A worn tire is smaller than a new one of the same size, so speedometer error changes slowly over the life of the tread.
Tire size questions
- How do I read a tire size like 225/70R19.5?
- The 225 is the section width in millimetres. The 70 is the aspect ratio: the sidewall is 70 percent as tall as the tire is wide. R means radial construction, and 19.5 is the rim diameter in inches. Letters before the size, such as P or LT, show the tire type, and the numbers after it give the load index and speed rating.
- How is tire diameter calculated?
- Multiply the width by the aspect ratio to get the sidewall height: 225 mm × 0.70 is 157.5 mm. Add two sidewalls to the rim diameter, with the rim converted at 25.4 mm per inch: 19.5 × 25.4 + 2 × 157.5 gives 810.3 mm, about 31.9 inches. The result is a nominal size; the real tire varies by maker.
- How does tire size change the speedometer?
- The speedometer counts wheel turns. A tire with a larger diameter covers more distance per turn, so at an indicated 100 km/h you travel faster than 100. A smaller tire does the opposite. The difference in percent equals the difference in diameter, which the calculator shows when you compare two sizes.
- Why can't I enter 11R22.5?
- Sizes such as 11R22.5 or 11R24.5 give the width and rim but no aspect ratio, so the sidewall height cannot be worked out from the size alone. We did not find an official source that states the ratio those sizes imply. Use the overall diameter or revolutions per mile from the tire maker's data book instead.
- What are revolutions per mile used for?
- Revolutions per mile, or per kilometre, is how many times the tire turns to cover that distance. Speedometers, hub odometers and engine computers use it to turn wheel or driveshaft speed into distance. The calculator works it out from the nominal circumference; tire makers publish a measured figure under load, which is the one to set in the vehicle.