This tire size calculator compares two tire sizes and estimates the percent change in overall diameter. Enter width, aspect ratio, and rim diameter for tire 1 and tire 2. The result helps you see how a size swap may affect circumference and indicated speed.
It does not prove a tire fits your car. Load rating, wheel width, clearance, and manufacturer specs still rule.
Worked example
Example with defaults: 225/45R17 versus 245/40R18. Diameter 1 ≈ 634.3 mm. Diameter 2 ≈ 653.2 mm. Change ≈ 2.98 percent taller for the second size.
| Tire | Size | Approx diameter |
|---|---|---|
| 1 | 225/45R17 | ~634.3 mm |
| 2 | 245/40R18 | ~653.2 mm |
| Change | ~2.98% |
Speedometer effect in plain language
If overall diameter grows, the car travels farther per wheel turn. The speedometer can read lower than true speed. If diameter shrinks, the opposite can happen. This page gives diameter percent, not a full calibrated speed table.
Fitment limits
Percent diameter is only one check. Rubbing on fenders, changing ABS behavior, and warranty questions are separate issues. Ask a tire professional before you buy.
Related tools: gas mileage calculator, fuel cost calculator, and percentage calculator.
Common mistakes
- Comparing sizes without matching units in each field
- Assuming any close percent is safe clearance
- Ignoring load and speed ratings on the sidewall
Plus sizing basics
Larger rims with lower aspect ratios try to keep overall diameter close. Always check the percent change rather than assuming the package matches.
Spare and winter sets
Seasonal wheels should stay near the original diameter so ABS and speedometer behavior stay predictable.
Professional confirmation
Use this percent as a conversation starter with a tire shop, not as the final fitment approval.
Practical planning tips
Write down the inputs you used so you can repeat the estimate later. Small changes in rate, depth, hours, or price can move the result more than people expect.
When you compare two options, change one variable at a time. That keeps the comparison honest and easy to explain to a partner, client, or lender.
Units and rounding
Keep units consistent across every field. Mixing inches with feet, litres with gallons, or monthly figures with annual figures is the most common source of wrong answers.
Round only at the end for ordering or payments. Early rounding in each step can stack into a surprising gap versus the live calculator.
What this estimate does not include
Taxes, delivery fees, labor, penalties, and one time service charges are often outside the core formula. Add those on paper when you build a real budget.
Local rules, code requirements, and lender overlays can still change the final decision even when the math is correct.
How to double check the result
Recalculate once with a pencil using the same formula shown on this page. If your manual result is close to the tool, your inputs are probably consistent.
If the two answers diverge, check the field labels again before you trust either number for spending or filing.
When to get a second opinion
Large purchases, structural work, medical or legal questions, and loan contracts deserve a qualified human review. Use this page to prepare better questions, not to replace that review.
Save a screenshot or note of the inputs and result date. That record helps when you talk with a contractor, accountant, or lender later.