Tire Size Calculator
Compare two tire sizes side by side: diameter, sidewall, circumference, revs per mile and speedometer error. Free, no sign-up, and it runs in your browser.
Dedicated page: /en/tire-size-calculator
Current tire (Tire 1)
New tire (Tire 2)
How to read a tire code: 205/55R16 means 205 mm wide, a sidewall height that is 55% of the width, radial construction, on a 16 inch rim. P means passenger, LT means light truck. Stay within 3% of the original diameter: more than that and the speedometer, odometer, gearing and wheel-arch clearance all change. This tool checks the maths only. It does not check load index, speed rating or rim width, so confirm those with the tire maker before you buy.
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Quick answer
A tire size calculator compares two tire sizes and works out the diameter, sidewall height, circumference and revs per mile for each. Enter your current size and the new size, and it shows the diameter difference as a percent, the speedometer error at every speed, and whether the new tire is safe to fit.
About this Tire Size Calculator
Compare two tire sizes side by side: diameter, sidewall, circumference, revs per mile and speedometer error. It is one of the Car and Travel on ToolUstad, and like every tool here it runs entirely in your browser: nothing you type, paste or open is uploaded to a server, and there is no sign-up or usage limit.
What is a tire size calculator?
A tire size calculator decodes the numbers on a tire sidewall and turns them into real measurements. A code like 205/55R16 looks cryptic, but it holds everything: the width in millimetres, the sidewall height as a percent of the width, the construction type, and the rim diameter in inches. From those three numbers the calculator works out the overall diameter, the sidewall height, the circumference, and how many times the tire turns in a mile. When you compare two sizes, it shows the difference as a percent, which is the number that decides whether the new tire is safe.
How tire size is worked out
The maths is simple and exact. Sidewall height is the width times the aspect ratio divided by 100, so a 205 mm tire at 55% has a sidewall of 112.75 mm. Overall diameter is the rim diameter plus twice the sidewall height, converted to inches: 16 inches plus two times 4.44 inches is 24.88 inches. Circumference is pi times the diameter, and revs per mile is 63,360 inches divided by the circumference. Every result on this page comes from those four steps, so you can check the working by hand.
Why the 3% rule matters
The tire industry rule is to stay within 3% of the original overall diameter, and within 1% if you can. Past 3%, the speedometer error becomes large enough to matter for speed limits, the odometer under-reads or over-reads distance, the gearing changes, and the tire may rub the wheel arch on bumps or at full steering lock. Anti-lock brakes and stability control are also calibrated to the original tire size. A bigger tire is not just a cosmetic change: it changes how the car measures speed, how far it travels per turn, and how much clearance it has.
Best practices
- Change all four tires together, or at least both tires on the same axle, with the same size, brand and tread pattern.
- Check the load index and speed rating of the new tire against the original: they must match or beat it, even if the diameter is perfect.
- Check rim width: every tire size has a rim width range, and a tire on too narrow or too wide a rim wears unevenly and handles badly.
- Check clearance at full steering lock and with the suspension compressed, not just parked straight.
Common mistakes
- Changing only the width and forgetting the aspect ratio: a 225/55 is much taller than a 205/55, not just wider.
- Trusting the speedometer after a size change: it is off by the same percent as the diameter.
- Ignoring the load index: a tire with the right diameter and a lower load index is the wrong tire.
- Mixing sizes across an axle, which confuses stability control and wears the differential.
Learn: how to use the Tire Size Calculator
- Enter your current tire size from the sidewall, for example 205/55R16.
- Enter the new size you are thinking of buying, for example 225/45R17.
- Read the diameter difference first: within 3% is safe, within 1% is ideal.
- Check the speedometer table to see your true speed at each indicated speed, then look at the compatible sizes list.
Features
- Old vs new size comparison with diameter, sidewall, circumference and revs per mile
- Diameter difference as a percent, with a fit warning outside the 3% safe range
- Speedometer error table from 20 to 100 mph, plus the odometer effect
- Visual diameter comparison drawn to scale
- Compatible sizes list: every common size within 3% of your original diameter
- Tire code decoder: width, aspect ratio, construction and rim explained
- Ground clearance change from the diameter difference
Worked example
A 205/55R16 tire is 24.88 inches across, with a 4.44 inch sidewall and 811 revs per mile. A 225/45R17 is 24.97 inches across, with a 3.99 inch sidewall and 808 revs per mile. The diameter difference is only 0.38%, so it is an ideal fit. At 60 mph on the speedometer the true speed is 60.2 mph, and ground clearance rises by 0.05 inches.
Frequently asked questions
How do I read a tire size like 205/55R16?
205 is the width in millimetres, 55 is the aspect ratio, the sidewall height as a percent of the width, R means radial construction, and 16 is the rim diameter in inches. A P before the numbers means passenger, LT means light truck.
How much can I change my tire size?
Stay within 3% of the original overall diameter. More than that and the speedometer and odometer read wrong, gearing changes, and the tire may rub the wheel arch. Within 1% is ideal, and this calculator flags both bands.
What happens to my speedometer with bigger tires?
A bigger tire covers more ground per turn, so the speedometer reads lower than your true speed. With a tire 3% larger, 60 mph on the dial is really 61.8 mph. The table on this page shows the true speed at every indicated speed.
Does a bigger tire change my ground clearance?
Yes, by half the diameter change. A tire 1 inch larger in diameter lifts the car by half an inch. That helps on rough tracks but raises the centre of gravity slightly.
Do I need to check anything besides diameter?
Yes. Load index and speed rating must match or beat the original tire, the rim width must suit the new tire, and the tire must clear the suspension and wheel arch at full steering lock. This tool checks the maths only.
Why does the revs per mile number matter?
It is how the car measures speed and distance. Fewer revs per mile means the odometer under-reads: with a tire 3% larger, 100 miles on the odometer is really 103 miles. It also shifts gearing slightly taller.
Can I fit a wider tire on the same rim?
Often, within one or two sizes. A 205 mm tire on a 6.5 inch rim can usually go to 215 or 225 mm, but the rim width, the wheel arch and the suspension all set limits. Check the tire maker fitment guide for your exact rim.
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