Tire Size Calculator
Compare two tire sizes: diameter, circumference and speedo error.
Enter your values
Enter the original width in mm.
Enter the original aspect ratio in %.
Enter the original rim in in.
Enter the new width in mm.
Enter the new aspect ratio in %.
Enter the new rim in in.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- New overall diameter
- 28.57 in
- Original diameter
- 28.52 in
- Diameter change
- 0.06 in (0.21%)
- Circumference (new)
- 89.77 in
- Revolutions per mile
- 706
- Speedo at indicated 60 mph
- 60.1 mph actual
- Fitment
- Within the usual ±3% safe window
Quick answer
Overall tire diameter = rim diameter + 2 × (section width × aspect ratio ÷ 100), converted to inches. A larger diameter makes the speedometer read low.
Diameter = rim + 2 × (width × aspect ÷ 100) ÷ 25.4
At a glance
| What it does | Compare two tire sizes: diameter, circumference and speedo error. |
|---|---|
| Category | Auto |
| Inputs needed | Original width, Original aspect ratio, Original rim, New width, New aspect ratio, New rim |
| Main output | New overall diameter |
| Formula | Diameter = rim + 2 × (width × aspect ÷ 100) ÷ 25.4 |
| Cost | Free — no sign-up, no download |
How to use the Tire Size Calculator
- 1Read your current size off the sidewall, e.g. 225/65R17.
- 2Enter width, aspect ratio and rim for the original and the new tire.
- 3Check the diameter change stays inside ±3% to keep the speedometer and ABS accurate.
Inputs explained
- Original width(mm)
- Enter the original width in mm.
- Original aspect ratio(%)
- Enter the original aspect ratio in %.
- Original rim(in)
- Enter the original rim in in.
- New width(mm)
- Enter the new width in mm.
- New aspect ratio(%)
- Enter the new aspect ratio in %.
- New rim(in)
- Enter the new rim in in.
Worked example
Using the values the calculator loads with:
Inputs
- Original width225 mm
- Original aspect ratio65 %
- Original rim17 in
- New width245 mm
- New aspect ratio60 %
- New rim17 in
Results
- New overall diameter28.57 in
- Original diameter28.52 in
- Diameter change0.06 in (0.21%)
- Circumference (new)89.77 in
- Revolutions per mile706
- Speedo at indicated 60 mph60.1 mph actual
- FitmentWithin the usual ±3% safe window
Frequently asked questions
How much tire size change is safe?
Most manufacturers accept an overall diameter within ±3% of stock. Beyond that, the speedometer, odometer, ABS and traction control all read incorrectly.
Does a wider tire change the diameter?
Yes — the sidewall height is a percentage of the width, so a wider tire with the same aspect ratio is also taller.
What do I need to enter into the Tire Size Calculator?
Just 6 values: original width, original aspect ratio, original rim, new width, new aspect ratio, new rim. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Tire Size Calculator work out the answer?
Overall tire diameter = rim diameter + 2 × (section width × aspect ratio ÷ 100), converted to inches. A larger diameter makes the speedometer read low. It applies the formula Diameter = rim + 2 × (width × aspect ÷ 100) ÷ 25.4 and shows the working so you can check each step by hand.
Is the Tire Size Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.