Tire Size FAQ
Short answers about tire sizes: what the sidewall code means, how size changes affect your speedometer, what load index and speed ratings are, and why you should check your owner’s manual, the door placard or a tire shop before changing size. The calculators do the maths for any size.
Questions and answers
Is tiresizecalculator.app free?
Yes. Every calculator on the site is free and needs no account. There is no advertising at the moment. The site is published by a small Swedish company, named at the bottom of every page.
Is anything I type sent to a server?
No. All calculations run in your browser. If you copy a link, the sizes are written into the address so the page can show the same comparison, but nothing is stored by us.
Can I fit a different tire size on my car?
Possibly, but it depends on your vehicle, wheels and local rules. Always check your vehicle owner’s manual, the tire placard in the door jamb, or a tire shop before changing size.
Will bigger tires change my speedometer?
Yes. A taller tire makes the speedometer show less than your actual speed by the same percentage as the diameter change. The speedometer calculator shows the actual speed at each reading.
What do load index and speed rating mean?
The load index is a code for the maximum load per tire; 94 means 670 kg. The speed symbol is a code for the maximum speed under standard conditions; W means 270 km/h.
How accurate are the calculations?
They are exact for the nominal size code. Real tires differ slightly by make, model, rim width and tread wear, so use the tire maker’s data for precise dimensions.
What is a good percentage difference?
There is no single safe number for all vehicles. The tool marks up to 3% green and over 5% red as our own guide to the size of the change, not as a legal limit.
Does changing tire size affect inspection or insurance?
It can, and the rules differ by country and state. Ask your inspection authority or insurer what applies to your vehicle before you change size.
What is the difference between P-metric and Euro-metric?
P-metric sizes (with P in front) follow the US TRA standard; sizes without a prefix usually follow the European ETRTO standard. The dimensions are calculated the same way, but load capacities can differ.
What is 33x12.50R15 in metric?
The closest metric size is about 315/75R15, which is about 0.6 inches taller. It is not an exact match, so compare the real diameter and width of the models you consider.
Still unsure?
Use the calculator, read how to read a tire size, or write to info@tiresizecalculator.app.
Last updated: October 15, 2026