Tire Size CalculatorNEW
Compare old and new tire sizes, diameter, width and speedometer error
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AllFreeTool is a free online tire size calculator that converts between modern tire codes such as 225/55R17 and old-fashioned sizes like 7.00-16. It shows the real sidewall height, overall diameter, circumference, and how much your speedometer will read wrong once you change size. You can also compare the original and the new size side by side. All of it runs locally in your browser.
It is useful before you order replacement tires, when comparing a cheaper alternative size, or when you want to understand why your speedometer reads higher than your true speed. It also converts an old bias-ply size into the closest modern equivalent, which helps when restoring a classic vehicle or reading an old manual.
Why use Tire Size Calculator on AllFreeTool?
Both Size Formats
Enter a modern code such as 225/55R17 or an old-style size such as 7.00-16.
Real Dimensions
See the sidewall height, the overall diameter and the rolling circumference in real units.
Speedometer Error
Learn the percentage difference between the indicated and the actual speed after a change.
Side-by-Side Compare
Put the original and the new specification together to compare the differences.
How to use Tire Size Calculator
Pick the Format
Choose the modern or the old-style format, then fill in width, ratio and rim diameter.
Calculate
Press convert to see the dimensions and the speedometer error for that size.
Compare Sizes
Enter the new size to see both specifications listed together for comparison.
Common questions
How do I read a modern tire size code?＋
The code has three numbers: the section width in millimetres, the aspect ratio as a percentage of that width, and the rim diameter in inches, with R meaning radial. So 225/55R17 is a 225 millimetre wide tire, a sidewall height of 55 percent of the width, on a 17 inch rim.
Why does my speedometer read differently after a size change?＋
The speedometer assumes a specific overall tire diameter, so a larger tire makes the needle read lower than your real road speed, and a smaller tire makes it read higher. The tool calculates the percentage error for you and also reports what your true speed is when the gauge shows 100.
Can I convert an old tire size to a modern one?＋
Yes. The old format such as 7.00-16 describes a bias-ply tire where the first number is the section width in inches and the second is the rim diameter. The calculator converts that into the closest modern equivalent so you can order a radial tire for an older vehicle.
Does the tool check my values for errors?＋
It does. If the numbers fall outside sensible ranges, for example a sidewall ratio that is far too large for the rim, the calculator refuses to produce a result and asks you to check the input. This avoids quietly giving you a wrong conversion based on a typo.