Tire Size Calculator
Decode a tire size like 205/55R16 into its real diameter, and compare two sizes to see how much a change affects your rolling diameter and speedometer reading — vital when fitting non-standard tires.
How to use this calculator
- Read your tire size off the sidewall: width / aspect ratio R rim (e.g. 205/55R16).
- Enter it as the current tire.
- Enter a new size to compare diameters and see the speedometer error.
Formula used
A tire like 205/55R16 means 205 mm wide, sidewall = 55% of the width (112.75 mm), on a 16-inch (406.4 mm) rim. Overall diameter = rim + two sidewalls. Aspect ratio is a percentage of width, not a fixed height — so wider tires with the same ratio are taller.
Example calculation
205/55R16: sidewall = 205 × 0.55 = 112.75 mm, diameter = 406.4 + 225.5 = 631.9 mm.
Switch to 225/45R17 (634.3 mm) and the diameter changes only +0.38% — a near-perfect "plus-size" that keeps the speedo accurate while fitting a bigger rim.
Why rolling diameter matters
The three numbers on a tire encode its shape, but the figure that matters for changing tires is the overall rolling diameter, because your speedometer, odometer, ABS and traction control are all calibrated to it. Fit tires with a larger diameter and the car travels further per wheel revolution than the electronics expect, so the speedo under-reads (you're going faster than it shows) and the odometer under-counts; a smaller diameter does the reverse.
This is why "plus-sizing" — going to a bigger rim — is done carefully: you drop the aspect ratio to keep the total diameter almost unchanged (a wider, lower-profile tire on a bigger wheel). Staying within about ±3% of original keeps the speedometer legal and accurate and avoids rubbing on the bodywork or suspension. Beyond that, you risk clearance problems and confused electronic systems.
Why use this calculator?
- Decode any tire size into its real diameter and sidewall height.
- Check the speedometer error before fitting a different size.
- Plan a plus-size that keeps diameter within the safe ±3%.
Frequently asked questions
What do the numbers on a tire mean?
In 205/55R16: 205 is the width in mm, 55 is the aspect ratio (sidewall height as a % of width), R means radial, and 16 is the rim diameter in inches. Together they set the overall tire size.
Will bigger tires mess up my speedometer?
If they change the overall diameter, yes. A larger diameter makes the speedo read low (you're actually going faster); smaller reads high. Keep the change within about ±3% to stay accurate and legal — this calculator shows the exact effect.
What is plus-sizing?
Fitting a larger-diameter rim with a lower-profile tire so the overall diameter stays nearly the same — e.g. going from 205/55R16 to 225/45R17. It improves looks and handling while keeping the speedometer accurate and avoiding clearance issues.