How to use this calculator
- Enter the old tire size. Enter old section width, aspect ratio and rim diameter.
- Enter the new tire size. Enter the proposed tire section width, aspect ratio and rim diameter.
- Set indicated speed. Enter the speedometer reading you want to compare.
- Check the percent change. Review diameter change and true speed before checking physical clearance.
How it works
Tire notation mixes units. In a size like 225/45R17, the 225 is section
width in millimetres, the 45 is sidewall height as a percentage of width, and the
17 is rim diameter in inches. The calculator uses
diameter = rim(in) x 25.4 + 2 x width(mm) x aspect/100.
Circumference is pi x diameter, and revolutions per
kilometre or mile are distance divided by circumference.
The percent diameter change also estimates speedometer change. A larger tire travels farther per wheel revolution, so true road speed is higher than indicated. After a tire size change, carry the new diameter into the engine RPM calculator and check wheel fitment with the wheel offset calculator.
Worked example
Verified against the live calculator
A 225/45R17 tire has a diameter near 634 mm. Switching to 245/40R18 gives about 653 mm, a change of roughly 3.0%. At an indicated 100 km/h, the vehicle is actually travelling about 103 km/h before tire-load effects.
Frequently asked questions
How do you calculate tire diameter?
For a tire like 225/45R17, sidewall height is 225 mm x 45% = 101.25 mm. Tire diameter is rim diameter in mm plus two sidewalls: 17 x 25.4 + 2 x 101.25.
What tire size change is usually acceptable?
A common screening rule is to keep overall diameter within about 3% of the original tire. Larger changes can affect speedometer reading, gearing, ABS/ESC calibration and clearance.
Why does the calculator ask for speed?
It uses the old-to-new diameter ratio to show true road speed at the entered indicated speed.
Is tire diameter exactly the same as rolling diameter?
No. Actual rolling radius depends on load, pressure, tire construction, tread depth and speed. Use manufacturer revs-per-mile data for precise calibration.
Method & assumptions
- Uses nominal unloaded tire geometry from section width, aspect ratio and rim diameter.
- Does not check fender, strut, spring perch, brake, load rating, ABS/ESC or legal clearance requirements.
- For final gearing or speedometer calibration, use measured rolling radius or manufacturer revs-per-mile data.