Tire size calculator (compare sizes and rims)
Compare two tire sizes, like 185/65 R15 and 195/55 R16: diameter, sidewall height, circumference, revolutions per km and the speedometer error when you change rims or tires.
How to useCompare tire sizes
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Tire size calculator (compare sizes and rims): what it is for
Type your current tire size and the new one, like 185/65 R15 and 195/55 R16. The tool calculates the sidewall height, overall diameter, circumference and revolutions per kilometer of each, shows the difference in percent, warns if it goes beyond ±3% and tells you your real speed when the dash shows a given value. A scale drawing compares both tires.
The calculations run in your browser and are an estimate; always check your vehicle manual.
How to use
- Type the current and new sizes.
- Enter a speedometer reading.
- See the difference and the speedometer error.
When to use it
- Change your wheel size.
- Choose an equivalent tire.
- Understand the size written on the tire.
Frequently asked questions
How do I read a tire size?
In 205/55 R16, 205 is the width in millimeters, 55 is the sidewall height as a percentage of the width (aspect ratio) and 16 is the rim diameter in inches. The R stands for radial construction.
How much can the diameter change when I switch tires?
The most common advice is to keep the overall diameter difference within about 3% so you don't affect the speedometer, odometer, ABS and traction control, and so the tire doesn't rub the wheel well. Also check your vehicle manual.
Why does the speedometer change?
The speedometer counts wheel revolutions. A bigger tire travels farther each turn, so the car goes faster than the dash shows. With a smaller tire it's the opposite.
Is it really free?
Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.