Tire Size Speedometer Calculator

Tire Size Speedometer Calculator

When you fit a different tire size than the one your car shipped with - a common move when upgrading wheels or "plus sizing" for looks - the overall rolling diameter of the tire changes. Your speedometer and odometer are calibrated for the original tire's diameter, so a different diameter means they no longer read your true speed and distance correctly.

This calculator works out the exact size of that error: how much your speedometer under- or over-reads, what your actual speed is at a given indicated speed, and how your odometer's distance count is affected.

How it's calculated

A tire's size is written as three numbers, e.g. 205/55R16 - width in millimeters, aspect ratio as a percentage of that width, and rim diameter in inches. The overall rolling diameter combines all three:

$$D_{mm} = (rim_{in} \times 25.4) + 2 \times \left(width_{mm} \times \frac{aspect}{100}\right)$$

Once you have the original and new diameters, your true speed and distance scale with the ratio between them:

$$speed_{actual} = speed_{indicated} \times \frac{D_{new}}{D_{original}}$$ $$odometer_{display\ per\ 100\ km} = 100 \times \frac{D_{original}}{D_{new}}$$

If the new tire's diameter is larger than the original, your wheel covers more ground per rotation than the speedometer assumes, so it under-reads your true speed and the odometer under-counts distance. If the new diameter is smaller, both over-read instead.

How to use it

1. Enter your original tire's width (mm), aspect ratio (%), and rim diameter (in) - these three numbers are printed on the tire's sidewall, e.g. 205/55R16.
2. Enter the same three numbers for the new tire size you're considering or have fitted.
3. Enter a speedometer reading (in km/h) you want to check the true speed for.
4. Submit to see the diameter change, your actual speed at that reading, a plain-language summary, and a table comparing indicated vs. actual speed across a range of common speeds.

Worked example

Going from a stock 205/55R16 to a lower-profile 235/40R18:

• Original diameter: 16 × 25.4 + 2 × (205 × 0.55) = 631.9 mm
• New diameter: 18 × 25.4 + 2 × (235 × 0.40) = 645.2 mm
• Diameter change: +2.10%

At an indicated 100 km/h, your actual speed is 100 × (645.2 / 631.9) ≈ 102.1 km/h - the speedometer under-reads by about 2.1%. Over an actual 100 km drive, the odometer would show only about 97.9 km, since it under-counts distance by the same ratio.

These figures update automatically for whatever original and new tire sizes and speedometer reading you enter.