Compare your current tire size against a new one
Enter the width, aspect ratio and rim diameter of the tire fitted to your car and of the one you are considering. Overall diameter, circumference, revolutions per kilometer, the change in ride height and the speedometer error are all recalculated the moment you change a number, along with a verdict on whether the difference is still safe.
Current Tire
- Width
- 215 mm
- Sidewall
- 118.25 mm
- Diameter
- 668.30 mm
- Circumference
- 2099.5 mm
- Revs / km
- 476.3 rev/km
New Tire
- Width
- 225 mm
- Sidewall
- 112.50 mm
- Diameter
- 656.80 mm
- Circumference
- 2063.4 mm
- Revs / km
- 484.6 rev/km
Comparison Result
- Width
- +10 mm (+4.65%)
- Sidewall
- -5.75 mm (-4.86%)
- Diameter
- -11.50 mm (-1.72%)
- Circumference
- -36.1 mm (-1.72%)
- Revs / km
- +8.3 rev/km (+1.75%)
- Ground Clearance
- -5.75 mm
- Speedometer Error
- -1.72%
Current / New
Speed Table
| Indicated speed (km/h) | 20 | 40 | 60 | 80 | 100 | 120 | 140 |
|---|---|---|---|---|---|---|---|
| Actual speed (km/h) | 19.66 | 39.31 | 58.97 | 78.62 | 98.28 | 117.94 | 137.59 |
| Deviation | -0.34 | -0.69 | -1.03 | -1.38 | -1.72 | -2.06 | -2.41 |
Safety recommendation
Safe to use
The diameter difference is within ±3% — safe for speedometer accuracy and ABS/traction control calibration.
Note
- Changing tire size may affect speedometer accuracy.
- A wider tire may affect grip, fuel economy, and ride feel.
- Please check compatibility with your wheels and vehicle before installing.
All Tools
Tire Size Calculator
Calculate and compare tire sizes
Plus / Minus Sizing
Find a matching size for a new wheel diameter
Wheel Fitment
Calculate offset and wheel position
Unit & Tire Converter
Convert units and tire sizes
Comparison & Reports
Compare multiple sizes and generate a report
Speedometer Error
Calculate speed reading deviation
Reading the numbers on the sidewall before you compare
A tire size such as 205/55R16 is three parts. The first is the tread width in millimeters. The second is the aspect ratio, the sidewall height as a percentage of that width rather than a fixed figure, so a 205/55 carries a 112.75 mm sidewall. The last is the rim diameter in inches. Enter all three for both the current and the new tire and the calculator works out the sidewall itself, converts the rim from inches to millimeters and adds a sidewall at the top and the bottom to reach the overall diameter. The inputs accept 100 to 400 mm of width, aspect ratios from 20 to 100 percent and rims from 10 to 30 inches, which covers cars, pickups, SUVs and metric motorcycle sizes. If the figures you have are in different units, keep the unit and pressure converter open alongside. Everything is computed on your own device as you type, nothing is sent to a server, and it opens straight from a phone browser with nothing to install.
What each result of the comparison means
With both sizes entered, the results come in two layers. The first is what each tire is on its own: sidewall height, overall diameter, circumference and revolutions per kilometer. Circumference is how far the tire rolls in one turn, pi multiplied by the overall diameter, and revolutions per kilometer is one million millimeters divided by that circumference, so a larger tire turns fewer times over the same distance. The second layer is the difference between the two: width, sidewall height, circumference and revolutions per kilometer, with overall diameter given in both millimeters and percent. Ride height change is half the diameter difference, since a diameter a millimeter larger lifts the body by half a millimeter. The last figure is speedometer error, which is exactly the diameter percentage, because the car counts speed from how fast the wheel turns rather than measuring the road. If you are changing wheels at the same time, read it alongside wheel offset and fitment.
How far off is still safe
The figure to read first is the diameter difference in percent, because that single number governs both the speed shown on the dial and the height of the car. The verdict comes in three bands: within plus or minus 3 percent is safe, 3 to 5 percent deserves caution, and beyond 5 percent is risky, with the dial drifting further the larger the gap grows. The speed table on this page spells the same thing out in real numbers, stepping from 20 to 140 kilometers per hour and putting the indicated reading beside the actual speed. The percentage error is constant at every step, but the further you drive the wider the gap becomes in kilometers per hour. A larger tire means the car is genuinely travelling faster than the needle claims, while a smaller one makes the needle read high. To look at that on its own, open the speedometer error table.
What the calculator cannot decide for you
Every figure here is worked out from the tire size alone, so it is exact about shape and distance and silent about everything else. The load index and speed symbol on the sidewall still need checking against what the carmaker specifies in the handbook or on the placard inside the driver's door. Clearance in the wheel well is the other blind spot, because the calculator does not know which car you drive: a size that clears the percentage band can still foul the liner or a suspension arm at full lock or full compression, which has to be measured on the car or checked with a fitter. There is no brand or model database behind it either, and no prices or shop stock. What it does do well is let you swap sizes quickly, with a button to flip the two tires and see the result the other way round, a reset, a save that keeps the pairs you compared in your browser, and a share button that copies a link to the result. To weigh several sizes at once, move to the multi-size comparison chart.