Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Diameter difference
2.98%
Original diameter (in)
24.97
New diameter (in)
25.72
True speed at indicated speed (mph)
61.8
New tire circumference (in)
80.79
Revolutions per mile
784
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter original width (mm).
- 2Enter original aspect ratio.
- 3Enter original rim (in).
- 4Enter new width (mm).
- 5Enter new aspect ratio.
- 6Enter new rim (in).
- 7Enter indicated speed (mph).
- 8Read your diameter difference on the right — it updates as you type.
- 9Hit Share to keep the scenario or send it to someone.
About this calculator
Changing tire size changes your effective final drive. More than about 3% difference in rolling diameter and your speedometer and odometer are meaningfully off.
Worked example
Using the values the calculator loads with:
Inputs
- Original width (mm): 225
- Original aspect ratio: 45
- Original rim (in): 17
- New width (mm): 245
- New aspect ratio: 40
- New rim (in): 18
- Indicated speed (mph): 60
Results
- Diameter difference: 2.98%
- Original diameter (in): 24.97
- New diameter (in): 25.72
- True speed at indicated speed (mph): 61.8
- New tire circumference (in): 80.79
- Revolutions per mile: 784.25
What each field means
Inputs
- Original width (mm)
- The original width (mm) used in the calculation. Starts at 225 so you have a working example on load.
- Original aspect ratio
- The original aspect ratio used in the calculation. Starts at 45 so you have a working example on load. Accepted range: 20–90.
- Original rim (in)
- The original rim (in) used in the calculation. Starts at 17 so you have a working example on load. Accepted range: 10–24.
- New width (mm)
- The new width (mm) used in the calculation. Starts at 245 so you have a working example on load.
- New aspect ratio
- The new aspect ratio used in the calculation. Starts at 40 so you have a working example on load. Accepted range: 20–90.
- New rim (in)
- The new rim (in) used in the calculation. Starts at 18 so you have a working example on load. Accepted range: 10–24.
- Indicated speed (mph)
- The indicated speed (mph) used in the calculation. Starts at 60 so you have a working example on load.
Results
- Diameter difference
- Returned as a percentage and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Original diameter (in)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- New diameter (in)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- True speed at indicated speed (mph)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- New tire circumference (in)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Revolutions per mile
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the tire size comparison calculator work?
Changing tire size changes your effective final drive. More than about 3% difference in rolling diameter and your speedometer and odometer are meaningfully off. The underlying maths is: Diameter (in) = Rim + 2 × (Width mm × Aspect% ÷ 25.4 ÷ 100).
What do I need to enter?
7 values: original width (mm), original aspect ratio, original rim (in), new width (mm), new aspect ratio, new rim (in), and indicated speed (mph). Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the diameter difference result mean?
It is returned as a percentage and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary.
Who is this tool for?
It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Tire Size Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/tire-size-comparison
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/tire-size-comparison" target="_blank" rel="noopener">Tire Size Calculator — RevenueLab</a> (2026).</p>
Source: [Tire Size Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/tire-size-comparison) (2026).