Tire Size & Speedometer Deviation Calculator
Use this calculator to compare the rolling circumferences of two tire sizes and calculate the speedometer deviation.

Tire Size & Speedometer Deviation Calculator
Choosing the correct tire size is crucial for safety, performance and fuel efficiency. When you change the wheel size, your speedometer may display inaccurate values. Our Tire Size & Speedometer Deviation Calculator helps you compare two tire sizes and understand the difference in rolling circumference and how it affects your speedometer reading.
What is Rolling Circumference?
The rolling circumference is the distance a wheel covers in one rotation. Larger tires have a greater circumference, meaning they cover more ground per rotation. If you switch to a bigger or smaller tire without adjusting your instrumentation, the speedometer reading will no longer reflect your actual speed.
How Speedometer Deviation Happens
A speedometer is calibrated for a specific rolling circumference. When you change your tire size, the wheel rotates more or less often per kilometre, which leads to a deviation. A larger tire results in a lower indicated speed, while a smaller tire results in a higher indicated speed. Local regulations allow only a small deviation; always check the legal tolerances in your country.
Using the Calculator
Enter your current tire size (e.g. 205/55 R16) and the new size you are considering. Click “Calculate tire size now” to see the difference in rolling circumference, the percentage deviation and how it impacts your speedometer reading. We also provide the recommended rim sizes and whether the combination is technically compatible.
Practical accessories like seperation nets, tyre Fit, car jacks, seat covers or floor mats help you change tires safely and keep your interior clean.
FAQ: Tire Size & Speedometer Deviation
What does the tire size mean?
Tire size codes (e.g. 205/55 R16) specify the width in millimetres, the aspect ratio (sidewall height as a percentage of width), the construction type (R = radial) and the rim diameter in inches. Understanding these numbers helps you choose compatible tires.
What is rolling circumference?
Rolling circumference refers to the perimeter of the tire in motion. It determines how far your vehicle travels with each wheel rotation. Larger circumference means fewer rotations per distance.
Why does my speedometer show a deviation after changing tires?
Because the speedometer is set to the factory tire size. If you install a larger size, the speedometer underestimates your speed; with a smaller size it overestimates. Use our calculator to estimate this deviation.
Which tire size fits my rim?
Only certain tire widths and profiles fit a given rim width and diameter. Check the rim width and choose from approved sizes listed by the manufacturer. Our calculator suggests matching rim sizes.
How does tire size affect fuel consumption?
Bigger tires increase rolling resistance and vehicle weight, which can raise fuel consumption. Smaller tires may reduce rolling resistance but might compromise handling. Balance performance, comfort and economy when choosing.
Is a speedometer deviation allowed?
Regulations usually allow a small tolerance (e.g. ∑0 to +10 %). Excessive deviation can lead to fines or safety issues. Always verify with local regulations after changing tires.
What happens if I use wheels/tires of a different size?
Changing tire size alters the rolling circumference. Smaller tires make the speedometer read higher, larger tires make it read lower. The speedometer is calibrated for the original tire size.
What is the legal tolerance for speedometer error?
Under EU law, the speedometer must never show less than the actual speed and may only over-read by up to 10% + 4 km/h. At 100 km/h real speed, it may show up to 114 km/h.
Do I need inspection approval when changing tire sizes?
Minor deviations usually only require a simple registration at the inspection authority. Major deviations require a full technical inspection and possibly speedometer recalibration.
Does tire size affect fuel consumption?
Yes. Larger tires are heavier and increase rolling resistance, raising fuel consumption. Smaller, lighter tires can reduce consumption.
Disclaimer (English)
This information is for general guidance only and does not replace professional advice.
Now it's time to calculate your tire Presur and find compatible original rims.










