Calculators
Tire Size Calculator
Type a tire size to see its real dimensions, then add a second size to compare diameters and see how far off your speedometer would read.
Drawn to scale. The dashed line marks the top of the original tire.
| 225/45R17 | 225/40R18 | Difference | |
|---|---|---|---|
| Section width | 225 mm8.86 in | 225 mm8.86 in | 0 mm0.00% |
| Sidewall height | 101 mm3.99 in | 90 mm3.54 in | −11 mm−11.11% |
| Rim diameter | 432 mm17.00 in | 457 mm18.00 in | +25 mm+5.88% |
| Overall diameter | 634 mm24.97 in | 637 mm25.09 in | +3 mm+0.46% |
| Circumference | 1,993 mm78.45 in | 2,002 mm78.81 in | +9 mm+0.46% |
| Revolutions per mile | 808 | 804 | −4−0.46% |
| Revolutions per km | 502 | 500 | −2−0.46% |
+0.46%
overall diameter difference
Within the commonly cited ±3% guideline. This is a rule of thumb, not a legal limit or a fitment approval. Check the vehicle maker’s approved sizes, load index, speed rating and clearance.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 20.1 mph | +0.1 mph |
| 30 mph | 30.1 mph | +0.1 mph |
| 45 mph | 45.2 mph | +0.2 mph |
| 60 mph | 60.3 mph | +0.3 mph |
| 70 mph | 70.3 mph | +0.3 mph |
| 80 mph | 80.4 mph | +0.4 mph |
Assumes the speedometer was accurate on the original size. Actual = shown × new diameter ÷ original diameter. A taller tire makes the car go faster than shown; a shorter one slower. The odometer is off by the same percentage.
Nominal dimensions from the size code; real tires vary slightly by brand and load. Processed on your device — nothing is sent anywhere.
Works in your browser. No account needed.
Frequently asked questions
What do the numbers in a tire size like 225/45R17 mean?
225 is the section width in millimeters. 45 is the aspect ratio: the sidewall height is 45% of the width, here 101.25 mm. R means radial construction, and 17 is the rim diameter in inches. A leading P marks a P-metric passenger tire and LT a light truck tire; ZR indicates a radial built for high speeds; a trailing code such as 94Y is the load index and speed rating, which do not change the dimensions.
How is the overall tire diameter calculated?
Sidewall height = width × aspect ratio ÷ 100. Overall diameter = rim diameter × 25.4 + 2 × sidewall height, in millimeters. For 225/45R17 that is 17 × 25.4 + 2 × 101.25 = 634.3 mm, or about 24.97 inches. Circumference is π × diameter, and revolutions per mile is 1,609,344 mm divided by the circumference.
How much does a bigger tire change my speedometer?
The speedometer counts wheel revolutions, so it is off by the same percentage as the diameter. True speed = indicated speed × new diameter ÷ original diameter. Going from 205/55R16 (631.9 mm) to 215/60R16 (664.4 mm) is 5.1% taller, so at an indicated 60 mph you are really doing about 63.1 mph. The odometer under-counts distance by the same proportion.
Is the ±3% diameter rule a legal limit?
No. Staying within about 3% of the original overall diameter is a rule of thumb widely repeated by tire retailers to keep speedometer, ABS and stability control readings close to what the car expects. It is not a regulation, and it does not guarantee that a size fits. Always check the sizes approved by the vehicle maker, the load index and speed rating, and physical clearance to suspension and bodywork.
Why do real tires measure slightly differently from the calculation?
The size code gives nominal dimensions. Each manufacturer’s actual width and diameter vary by a few millimeters with tread depth, rim width and design, and a loaded tire flattens where it meets the road, so its rolling circumference is slightly smaller than the free circumference shown here. For exact figures, check the manufacturer’s specification sheet, which often lists revolutions per mile measured under load.
Can I enter flotation sizes like 33x12.50R15?
Yes. In flotation sizing the first number is the overall diameter in inches, the second is the section width in inches and the last is the rim diameter. The calculator converts them to millimeters and derives the sidewall height as half the difference between overall and rim diameter, so you can compare a flotation size directly against a metric one.
About this tool
This tire size calculator decodes the size printed on a sidewall into the dimensions that matter: section width, sidewall height, rim diameter, overall diameter, circumference, and revolutions per mile and per kilometer. It reads P-metric and Euro-metric passenger sizes, light truck sizes with an LT prefix, ZR and run-flat codes, and flotation sizes such as 31x10.50R15, and it ignores the load index and speed rating on the end because they do not affect the geometry.
The math is standard. Sidewall height is the section width multiplied by the aspect ratio, overall diameter is the rim diameter converted to millimeters plus two sidewalls, circumference is π times the diameter, and revolutions per mile is the length of a mile, 1,609,344 mm, divided by the circumference. These are nominal values from the size code; real tires from different brands vary by a few millimeters, and a tire under load rolls on a slightly smaller effective circumference, which is why manufacturers’ published revolutions per mile are usually a little higher than the geometric figure.
Add a second size to plan an upgrade, a plus-size wheel change or a winter set. Both tires are drawn side by side to scale, every dimension gets a millimeter and percentage difference, and the speedometer table shows the true speed behind several indicated speeds. Because a speedometer counts wheel revolutions, a tire that is 3% taller makes the car travel 3% faster than shown, and the odometer records 3% less distance than you actually drive.
The diameter difference is compared with the ±3% guideline that tire shops commonly quote. Treat it as guidance only: it is not a legal limit, and it says nothing about whether the tire clears the suspension, arches and brakes, or meets the load index and speed rating your vehicle requires. In countries that apply UN Regulation No. 39, a speedometer must never show less than the true speed, which is one more reason to avoid fitting noticeably taller tires. The vehicle maker’s approved sizes and a tire professional have the final word; everything here runs in your browser and nothing is sent anywhere.