What this calculator does
A tire size like 225/45R18 encodes three numbers: 225 mm of tread width, a sidewall height that is 45% of that width, and an 18 inch rim. The rolling diameter is twice the sidewall plus the rim, and it is the only figure that matters for the speedometer.
Because the speedometer is calibrated to the original diameter, changing tires makes it read wrong by the same percentage. A 225/45R18 replaced by 235/40R19 is 1.65% larger, so an indicated 100 km/h is really 101.7. The usual guidance is to stay within about 3% of the original, which keeps the error inside most speedometer tolerances.
The formula
The sidewall height is the section width multiplied by the aspect ratio as a percentage. The overall diameter is twice that sidewall plus the rim diameter converted to millimetres at 25.4 mm per inch. The percentage difference between the two diameters is the speedometer error, and applying it to an indicated speed gives the true one.
| Term | Meaning |
|---|---|
| Section width | The first number, tread width in millimetres. |
| Aspect ratio | The second number, sidewall height as a percentage of the width. Lower means a shorter sidewall. |
| Rolling diameter | Overall tire diameter, which is what the speedometer calibration depends on. |
| Plus sizing | Going to a larger rim with a lower profile to keep the overall diameter roughly the same. |
The inputs explained
| Field | What to enter |
|---|---|
| Current tire width (mm) | Original tire section width in millimetres. |
| Current aspect ratio (%) | Original aspect ratio, the second number in the size. |
| Current rim diameter (in) | Original rim diameter in inches. |
| New tire width (mm) | New tire section width. |
| New aspect ratio (%) | New aspect ratio. |
| New rim diameter (in) | New rim diameter. |
| Speedometer reading to check (km/h) | An indicated speed to convert, for seeing the error in practical terms. |
When to use it
Checking a tire size before buying
A diameter change beyond about 3% affects the speedometer, the odometer and on some cars the traction control, so it is worth checking before ordering.
Plus sizing to a larger rim
Going up a rim size while dropping the aspect ratio keeps the diameter close, and the calculation confirms how close.
Explaining an odometer discrepancy
A car reading differently from a GPS usually has a tire diameter that does not match the calibration, and the percentage shows by how much.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does aspect ratio change the diameter?
A fixed new width and rim with the aspect ratio varying.
| New aspect ratio | Diameter change | New tire diameter | True speed at an indicated 100 km/h |
|---|---|---|---|
| 235/35R19 | -1.91% | 647.1 mm (25.48 in) | 98.1 km/h |
| 235/40R19 | 1.65% | 670.6 mm (26.40 in) | 101.7 km/h |
| 235/45R19 | 5.21% | 694.1 mm (27.33 in) | 105.2 km/h |
| 235/50R19 | 8.78% | 717.6 mm (28.25 in) | 108.8 km/h |
Questions
How much tire size change is acceptable?
Within about 3% of the original rolling diameter is the usual guidance. Beyond that the speedometer and odometer read noticeably wrong, and on some vehicles the ABS and traction control systems, which infer wheel speed, can be affected.
Does a bigger tire make the speedometer read high or low?
Low. A larger tire covers more ground per revolution, so the car is going faster than the speedometer thinks. A smaller tire has the opposite effect and makes the speedometer over-read.
What does 225/45R18 mean?
225 mm of section width, a sidewall that is 45% of that width so about 101 mm, and an 18 inch rim. The R means radial construction. The overall diameter is therefore about 659 mm.
Does plus sizing change the ride?
Yes, noticeably. A lower profile tire has less sidewall to absorb bumps, so the ride is firmer and the wheel is more vulnerable to pothole damage. Handling response usually improves, which is the trade being made.
For fuel cost over a journey, see the trip fuel cost calculator. For the full running cost of a car, see the cost per mile calculator.