How this calculator works
A tire size code like 225/45R17 packs three measurements into one string: 225 is the section width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of that width), and 17 is the wheel (rim) diameter in inches. This calculator turns two tire codes into overall diameter, circumference, sidewall height, and revolutions per mile or kilometer, then compares them directly.
Comparing tire sizes matters most when considering a 'plus-size' fitment (a larger wheel with a lower-profile tire) or checking whether a replacement tire is close enough to the original. Changing overall diameter changes how far the car travels per wheel rotation, which throws off the speedometer and odometer by the same percentage as the diameter change, since both instruments are calibrated against the original tire's rotation rate.
The formulas
Sidewall height (mm) = section width (mm) × aspect ratio (%) ÷ 100Overall diameter (mm) = wheel diameter (in) × 25.4 + 2 × sidewall height (mm)Circumference (mm) = overall diameter (mm) × πActual speed = indicated speed × (new tire diameter ÷ original tire diameter)The speedometer and odometer are calibrated to the vehicle's original tire diameter. Fitting a different-diameter tire changes actual travel distance per wheel rotation without changing what the speedometer displays, so both readings drift by the same ratio as the diameter change.
Worked example: 225/45R17
- Sidewall height = 225 × 45 ÷ 100 = 101.25mm.
- Overall diameter = 17 × 25.4 + 2 × 101.25 = 431.8 + 202.5 = 634.3mm.
- Circumference = 634.3 × π ≈ 1,992.7mm.
- Revolutions per mile = 1,609,344mm ÷ 1,992.7mm ≈ 807.6 revolutions.
Frequently asked questions
How much diameter difference between two tire sizes is considered safe?
A commonly cited rule of thumb among tire shops and enthusiasts is to keep the overall diameter within about 3% of the original size, to avoid meaningfully affecting the speedometer, ABS/traction control calibration, gearing, and ground clearance. This calculator shows you the exact percentage difference so you can judge against whatever tolerance your vehicle or local regulations call for.
Why does a bigger tire make my speedometer read low?
A larger-diameter tire covers more ground per wheel rotation than the tire the speedometer was calibrated for. Since the speedometer only measures rotation rate, it still reports the speed it would show with the original tire, understating your actual road speed. This calculator's 'actual speed' figures show how far off the display would run.
Does the aspect ratio number mean the same thing across different tire widths?
Aspect ratio is always a percentage of the section width, so the same aspect ratio number produces a taller sidewall on a wider tire. A 45-aspect 225mm-wide tire has a 101.25mm sidewall, while a 45-aspect 265mm-wide tire has a taller 119.25mm sidewall.
What is 'plus sizing' in the tire world?
Plus sizing means fitting a larger wheel with a lower-profile (smaller aspect ratio) tire while trying to keep the overall diameter close to the original, to preserve the speedometer, gearing and ground clearance while changing the car's look and handling feel. This calculator is a common way to check whether a proposed plus-size combination actually holds the diameter close enough.
Why do revolutions per mile matter?
Revolutions per mile (or kilometer) tell you how many times the wheel turns to cover that distance, which is what many transmission gearing, ABS wheel-speed sensor, and off-road crawl-ratio calculations are actually built around, rather than diameter alone.