About the Tire Size Calculator
This tire size calculator decodes metric tire sizes such as 225/45R17 and compares your current tires with a replacement size. For each tire it works out the overall diameter, sidewall height, section width, circumference and revolutions per mile, then shows the percentage difference in diameter and how far off your speedometer will read.
Use it when you are fitting larger wheels (plus-sizing), choosing all-terrain tires for a truck, or checking whether an alternative size will fit. A tire with a larger overall diameter turns fewer times per mile, so the speedometer and odometer under-read: when the dash shows 60 mph you are really going faster.
A common rule of thumb is to keep the new diameter within about ±3% of the original to avoid speedometer error, ABS and stability-control problems and rubbing. Always confirm clearance, load index and speed rating with your vehicle’s placard or a tire professional before buying.
With the default inputs, the diameter difference is 1.72%. Change any value above to recalculate instantly.
How to use the tire size calculator
- 1Read the size on your current tire sidewall, e.g. 225/45R17.
- 2Enter the width, aspect ratio and rim diameter for the current tire.
- 3Enter the same three numbers for the size you are considering.
- 4Check the diameter difference and actual speed at your chosen speedometer reading.
- 5Aim to stay within about ±3% and confirm fitment with a tire shop.
Formula and method
A metric tire size such as 225/45R17 gives the section width in millimetres (225), the aspect ratio — sidewall height as a percentage of width (45) — and the rim diameter in inches (17). The sidewall height is width × aspect ratio, converted to inches by dividing by 25.4. Overall diameter is the rim plus two sidewalls.
Circumference is π × diameter and revolutions per mile is 63,360 inches ÷ circumference. Because the speedometer counts wheel revolutions, the true speed equals the indicated speed multiplied by the ratio of the new diameter to the old one. Real tires vary slightly by brand and tread depth.
- Width
- Section width in mm
- Aspect%
- Sidewall height as a percent of width
- Rim
- Wheel diameter in inches
- D
- Overall tire diameter in inches
Worked examples
225/45R17 to 235/40R18 plus-one upgrade
The stock tire is 24.97 in tall and the 18-inch option 25.40 in, 1.72% larger. At an indicated 60 mph you would be travelling about 61.0 mph — within the ±3% guideline.
Truck: 265/70R17 to 285/75R17
Going from a 31.6-inch to a 33.8-inch tire is a 7% jump. The speedometer would read 65 mph when you are actually doing about 69.6 mph, so recalibration is strongly recommended.
205/55R16 to 225/45R17
This classic plus-one size keeps the overall diameter almost identical (+0.38%) while fitting a larger wheel and wider tread.
Lower-profile tire on the same rim
Dropping from a 60 to a 55 aspect ratio shrinks the tire by 3.2%, so the speedometer over-reads: 70 mph on the dash is really about 67.7 mph.
Frequently asked questions
How do I read a tire size like 225/45R17?+
225 is the section width in millimetres, 45 is the aspect ratio — the sidewall is 45% as tall as the tire is wide — R means radial construction, and 17 is the wheel diameter in inches. A load index and speed rating such as 94W usually follow.
How much bigger can I go on tire size?+
Most tire shops and manufacturers recommend keeping the new overall diameter within about 3% of the original. Larger changes affect speedometer accuracy, gearing, ABS and stability control, and can cause rubbing on suspension or fenders.
Do bigger tires make my speedometer wrong?+
Yes. A taller tire covers more ground per revolution, so the speedometer reads lower than your true speed and the odometer records fewer miles. A smaller tire causes the opposite. Many vehicles can be recalibrated by a dealer or with a programmer.
Can I put wider tires on my stock wheels?+
Often by one size (about 10 mm), as each tire size has an approved range of rim widths. Check the tire manufacturer’s approved rim width range and make sure there is clearance at full steering lock and suspension compression.
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