Transparent method
The formula
Worked example
Example inputs
How to interpret the result
A tire size swap that changes the overall rolling diameter throws off both the speedometer and the odometer simultaneously, in the identical direction, because both readings are calibrated against the original tire's specific revolutions-per-mile figure, and a genuinely different diameter changes how many actual revolutions cover each real mile driven.
Before you rely on it
What to check
Compare the calculated diameter difference between old and new tire sizes before installing a size that differs meaningfully from the original factory specification.
The common error
Where people go wrong with tire size comparison calculator
Assuming a tire size change only affects ride height or appearance. A meaningfully different overall diameter also throws off speedometer and odometer accuracy together, since both are calibrated against the original tire's specific size.
Sensitivity evidence
How current tire width (mm) changes the diameter difference
| Current tire width (mm) | Diameter difference | Actual speed at an indicated 60 mph |
|---|---|---|
| 180 | 10.9% | 66.5 |
| 203 | 6.2% | 63.7 |
| 225worked example | 2.1% | 61.2 |
| 248 | -1.9% | 58.8 |
| 270 | -5.5% | 56.7 |
Every input, tested
Which input moves the diameter difference most
| Input | Tested from | To | Diameter difference at each end | Swing |
|---|---|---|---|---|
| New wheel diameter (in) | 15 | 19 | -5.5% to 9.6% | 15.0% (731%) |
| Current wheel diameter (in) | 14 | 18 | 10.3% to -5.1% | 15.4% (750%) |
| Current tire width (mm) | 203 | 248 | 6.2% to -1.9% | 8.1% (396%) |
| Current aspect ratio | 54 | 66 | 6.3% to -1.9% | 8.2% (397%) |
| New tire width (mm) | 212 | 259 | -1.7% to 6.0% | 7.6% (372%) |
| New aspect ratio | 50 | 61 | -1.4% to 6.2% | 7.6% (372%) |
Two variables at once
Diameter difference by current tire width (mm) and current aspect ratio
| Current tire width (mm) \ Current aspect ratio | 48 | 60 | 72 |
|---|---|---|---|
| 180 | 19.2% | 10.9% | 3.7% |
| 203 | 14.8% | 6.2% | -1.2% |
| 225 | 10.9% | 2.1% | -5.5% |
| 248 | 7.1% | -1.9% | -9.6% |
| 270 | 3.7% | -5.5% | -13.2% |
Step by step
The worked example, input by input
| Input | Value used |
|---|---|
| Current tire width (mm) | 225 |
| Current aspect ratio | 60 |
| Current wheel diameter (in) | 16 |
| New tire width (mm) | 235 |
| New aspect ratio | 55 |
| New wheel diameter (in) | 17 |
| Diameter difference | 2.1% |
| Actual speed at an indicated 60 mph | 61.2 |
Where the inputs come from
Read the tire size from the sidewall in the form 225/60R16: the first number is the section width in millimeters, the second is the aspect ratio, the sidewall height as a percentage of the width, and the last is the wheel diameter in inches. Enter your current size and the new size you are considering. The tool computes the overall diameter of each tire, including two sidewalls and the wheel, so the two tires can be compared directly.
When the result stops applying
Load index and speed rating must meet the vehicle's requirements. The calculation uses nominal sizes, and tires vary by brand and wear. Air pressure and load change the rolling diameter slightly. It does not check offset or brake caliper clearance.
What to compare it with
Estimate speed accuracy on the speed converter, and compare running costs on the fuel cost and vehicle cost of ownership calculators. The unit conversion pages convert millimeters and inches, and the auto loan calculators cover the purchase. A tire shop can confirm fit and load rating for your vehicle.
People also ask
Frequently asked questions
What do the numbers on a tire mean?
For 225/60R16, 225 is the width in millimeters, 60 is the sidewall height as a percent of the width, R means radial construction and 16 is the wheel diameter in inches.
How much can tire diameter change safely?
Most guides suggest staying within about 3% of the original overall diameter, to keep the speedometer and gearing close to stock.
Will a larger tire make my speedometer read low?
Yes. A larger diameter covers more ground per revolution, so your true speed is higher than the speedometer shows.