Tire Size Calculator
Compare tire diameter, circumference, ride height and speedometer changes.
Results
Diameter difference (%)
| Results | Current tire | New tire |
|---|---|---|
| Sidewall height | 101.25 mm | 105.75 mm |
| Overall diameter | 685.1 mm | 694.1 mm |
| Circumference | 2,152.31 mm | 2,180.58 mm |
| Revolutions per km / mile | 464.62 rev/km | 458.59 rev/km |
- Diameter difference
- 9 mm
- Ride height change
- 4.5 mm
This calculator compares tire geometry only. It does not determine vehicle fitment, wheel compatibility, load index, speed rating, suspension interference, fender interference or manufacturer approval. Before fitting different tires, check the permitted specifications from the vehicle and tire manufacturers.
Formula · How it works
Formula
Sidewall height = Width × Aspect ratio / 100; Diameter = Rim diameter in inches × 25.4 + 2 × Sidewall height (mm); Circumference = π × Diameter; Diameter difference = New diameter − Current diameter; Ride height change = Diameter difference / 2; Actual speed = Indicated speed × New diameter / Current diameterSidewall height is width × aspect ratio / 100. Convert rim diameter to mm using 25.4 mm per inch, then add two sidewalls. Diameter difference (%) = (new diameter − current diameter) / current diameter × 100. Revolutions per km = 1,000,000 / circumference in mm; revolutions per mile = 1,609,344 / circumference in mm. Speedometer difference = estimated actual speed − indicated speed.
Worked example
Changing from 225/45R19 to 235/45R19 changes nominal diameter from 685.1 mm to 694.1 mm. The diameter increases by 9 mm, approximately 1.31%, and ride height increases by 4.5 mm. With an indicated speed of 100 km/h, estimated actual speed is 101.31 km/h, a difference of about +1.31 km/h.
How it works
A wider tire with the same aspect ratio has a taller sidewall. A larger overall diameter travels farther per revolution, so the estimated actual speed is higher at the same indicated speed. The speed comparison assumes the speedometer matched the current tire and uses nominal, unloaded tire geometry; actual rolling dimensions can differ.
Common questions
What does 225/45R19 mean?
225 is the nominal tire width in millimeters. 45 is the sidewall height as a percentage of width, so the sidewall is 101.25 mm. R denotes radial construction, and 19 is the rim diameter in inches.
How is tire diameter calculated?
Convert rim diameter from inches to millimeters and add twice the sidewall height. For 225/45R19, the calculation is 19 × 25.4 + 2 × 225 × 0.45 = 685.1 mm.
How does tire size affect speedometer readings?
Assuming the speedometer matched the current tire, multiply indicated speed by new diameter divided by current diameter. A larger new diameter gives a higher estimated actual speed; a smaller diameter gives a lower speed.
Does a larger tire increase ride height?
The nominal ride-height change is half the diameter difference. A 9 mm increase in diameter gives a 4.5 mm increase in radius. Loaded tire deflection and the vehicle's suspension can affect the actual change.
Can I change rim diameter?
You can compare a different rim diameter here, but the result does not establish compatibility. Check the tire's permitted rim sizes and the vehicle and tire manufacturers' specifications before changing wheels or tires.