How is this calculated?
Tire diameter (in) = rim diameter + 2 * (width mm * aspect ratio / 100 / 25.4) Circumference (in) = pi * diameter Revolutions per mile = 63360 / circumference Speedometer error % = (new diameter - old diameter) / old diameter * 100
Example: 265/70R17 has an approximate diameter of 31.61 inches. A 285/70R17 tire is about 32.71 inches, so the speedometer reads roughly 3.5% slow.
How do I use this calculator?
- Choose the unit or currency setting that matches your vehicle data.
- Enter the required vehicle, route, fuel, weight or loan values in the calculator form.
- Review inline warnings and correct any missing or negative inputs.
- Read the live result card for the primary answer and supporting totals.
- Use the worked example if you want to check the formula with sample values.
- Copy, share or print the results for comparison or record keeping.
What do the terms mean?
- Section width
- The tire width in millimeters, such as 265 in 265/70R17.
- Aspect ratio
- Sidewall height as a percentage of tire width.
- Rim diameter
- Wheel diameter in inches.
- Revs per mile
- How many tire rotations occur in one mile.
- Speedometer error
- Difference between indicated speed and actual road speed after a tire change.
What are real-world examples?
| Scenario | Inputs | Result | Notes |
|---|---|---|---|
| Stock SUV | 265/70R17 | 31.61 in | 638 rev/mi |
| Larger all-terrain | 285/70R17 | 32.71 in | 616 rev/mi |
| Sedan tire | 225/55R17 | 26.74 in | 754 rev/mi |
| Truck tire | 275/65R20 | 34.07 in | 592 rev/mi |
What tips improve accuracy?
- Keep diameter changes small if you want accurate speedometer and ABS behavior.
- Check wheel well clearance at full steering lock and suspension compression.
- Load range matters for trucks and towing, not only tire dimensions.
- Use manufacturer-approved sizes when possible.
- Recalibrate the speedometer for large tire changes.
- Match tire pressure to load and tire rating.
Did you know? EPA estimates commonly use 8.89 kg CO2 per gallon of gasoline and 10.18 kg CO2 per gallon of diesel for tailpipe carbon dioxide calculations.
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Frequently asked questions
Tire size calculator
A tire size calculator helps compare tire diameter, sidewall height, circumference, and speedometer change when a customer changes tire size. It reads numbers such as 225/60R17. The 225 is width in millimeters, 60 is sidewall height as a percent of width, and 17 is rim diameter in inches. The calculator is useful, but final fitment must also consider load rating, speed rating, wheel width, brake clearance, suspension clearance, and manufacturer recommendations.
Free online tyre size calculator
A good free online tyre size calculator should let you enter the original and new tyre sizes, then compare overall diameter, circumference, sidewall height, revolutions per mile or kilometer, and speedometer difference. The safest change is usually a small diameter difference, often around 3 percent or less, but the correct limit depends on the vehicle. Also check load index, speed rating, wheel width, fender clearance, and local roadworthiness rules.
How cycle tyre size calculator works
A cycle tyre size calculator works by using the tyre's labeled size or measured rolling circumference. Bicycle tyres may use ISO sizing, inches, or old naming systems, so the calculator converts them to diameter and circumference. For speedometers, the best method is rollout: mark the tyre, roll one full revolution with rider weight, and measure the distance. This gives more accurate speed and distance than relying only on printed tyre size.
How to motorbike tyre size calculates
Motorbike tyre size is calculated from width, aspect ratio, and rim size. For example, in 180/55R17, 180 is width in millimeters, 55 means sidewall height is 55 percent of width, and 17 is rim diameter in inches. Overall diameter is rim diameter plus two sidewalls. On motorcycles, never choose only by math. Load rating, speed rating, clearance, handling, ABS, traction control, and manufacturer-approved sizes are very important.
How tyre size conversion calculator works
A tyre size conversion calculator compares the original tyre with a new size. It calculates sidewall height, overall diameter, circumference, and percentage difference. If the new tyre is larger, the speedometer may read slower than actual speed; if smaller, it may read faster. It also helps estimate clearance changes. Still, a calculator cannot confirm safe fitment by itself. Always check vehicle placards, wheel width, load index, speed rating, and suspension clearance.