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
Is this calculator free to use?
Yes. The calculator runs in your browser, does not require an account and can be printed or copied for your records.
Can I use this for official tax, loan or compliance filing?
Use it as an estimate. For official filing, financing or enforcement decisions, verify the final numbers with your lender, tax authority, manufacturer rating label or compliance professional.
Why are my real-world results different?
Real vehicles vary with load, speed, terrain, temperature, tire condition, maintenance, driver behavior and measurement quality.
Does the calculator save my units?
Yes. Unit and currency selections that are marked as preferences are stored in localStorage on your device so the next visit starts with your preferred setting.
Can I share the result?
Yes. Use the Share button if your browser supports native sharing, or copy the permalink and result summary.
Does it work on mobile?
Yes. The layout stacks on small screens, keeps labels visible and recalculates as you type.
How accurate are the formulas?
The formulas are standard industry math for estimates. Accuracy depends on the input values and whether the assumptions match your vehicle and jurisdiction.
When was this page updated?
This page was last updated on 2026-05-01 for 2026 planning assumptions and source links.