Tire Size Calculator
Compare tire sizes for diameter, sidewall, circumference and speedometer error.
Inputs
Width/Aspect RimDiameter — e.g. 225/45R17
Tire 1 Diameter
24.972in
Tire 1 Diameter
634.3mm
Sidewall Height
101.3mm
Circumference
78.452in
Revolutions per Mile
807.63
Revolutions per km
501.84
Step by step
Sidewall height
225 × 45%
= 101.3 mm
Overall diameter
2 × 101.3 mm + 17 × 25.4
= 24.972 in (634.3 mm)
Circumference
π × 24.972
= 78.452 in
Revolutions per mile
63,360 ÷ 78.452
= 807.63 rev/mi
How it works
A metric tire like 225/45R17 means: 225 mm section width, 45% aspect ratio (sidewall = 45% of 225 = 101.25 mm), and 17-inch rim diameter. The overall diameter is twice the sidewall plus the rim. Circumference = π × diameter. Revolutions per mile = 63,360 in ÷ circumference. If you switch to a different size, your speedometer reads based on the old rev count; a larger tire makes the speedo under-read.
Formulas
Overall diameter
Diameter (in) = 2 × (width × aspect/100) / 25.4 + rim diameter
- W
- Section width (mm)
- A
- Aspect ratio (%)
- R
- Rim diameter (in)
Revolutions per mile
Revs/mile = 63,360 ÷ (π × diameter in inches)
- 63360
- Inches per mile
Frequently Asked Questions
What does the 'R' mean in a tire size?
The R stands for Radial construction — the internal cords run perpendicular to the direction of travel, from bead to bead. Virtually all modern passenger tires are radial.
How much speedometer error is acceptable?
In most countries, regulations permit speedometers to read up to ~10% high but never below actual speed. An error of 1–2% is essentially imperceptible. Beyond ±3% you may want to consider a speedometer recalibration or a closer size match.
Will a plus-one tire fit my car?
A plus-one fitment (larger rim, lower profile tire) should maintain nearly the same overall diameter to avoid clearance issues. This calculator shows the diameter change so you can stay within the ±3% guideline.
How do I read a tire size?
225/45R17: 225 = section width in mm; 45 = aspect ratio (sidewall height as % of width); R = radial; 17 = rim diameter in inches. You can find this on the tire sidewall, usually preceded by the load index and speed rating.