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Tire Size Calculator

Compare two tire sizes and see the speedometer error.

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Current tire

New tire

Side by side

MeasurementCurrentNewChange

Speedometer error

Speedometer showsActual speedDifference

Reading a tire size

In 225/45R17: 225 is the section width in millimetres, 45 is the sidewall height as a percentage of that width, R means radial construction and 17 is the rim diameter in inches. Because the sidewall is a proportion of the width, changing the width changes the overall diameter too — which is why plus-sizing needs a lower aspect ratio to compensate.

Diameter change

—

The new tire

Overall diameter—
Sidewall height—
Circumference—
Revolutions per mile—

Effects

Ground clearance—
Speedo at 60 mph—

—

Against the usual 3% diameter guideline

Staying within about 3 percent of the original overall diameter is the usual guidance. Beyond that, speedometer error, ABS and traction control calibration, and clearance at full lock all become real concerns. Always check the load and speed ratings as well as the size.

Quick Tool Overview

The Tire Size Calculator decodes a sidewall size into real dimensions and compares two sizes side by side — diameter, width, circumference and revolutions per mile — with the speedometer error a change would introduce.

What it does
Compares two tire sizes and their effects
You give it
Two sizes in width/aspect/rim format
You get back
Diameter, circumference and speedometer error
Best for
Changing wheel or tire size, and plus sizing
Calculators Free, unlimited use No sign-up Runs in your browser

About the Tire Size Calculator

Decode a tire size into its actual dimensions and compare two sizes side by side. Overall diameter, section width, sidewall height, circumference and revolutions per mile are all calculated, along with the speedometer error a change would introduce.

A size like 225/45R17 encodes three separate things: the section width in millimetres, the aspect ratio as a percentage of that width, and the rim diameter in inches. Because the sidewall is a proportion of the width rather than a fixed figure, changing the width changes the overall diameter too, which is exactly why plus-sizing needs a lower aspect ratio to compensate. Staying within about three percent of the original diameter is the usual guidance.

Key features of the Tire Size Calculator

Decodes the sidewall

Explains what each number in 225/45R17 actually means, rather than assuming you know.

Speedometer error

What your speedometer would read at a true 60 mph or 100 km/h with the new size.

The 3 percent guideline

Flags when a change takes overall diameter outside the usual safe range.

Ground clearance change

Half the diameter difference, which is what actually changes at the arch.

Revolutions per mile

The figure ABS and odometer calibration are based on.

How to use the Tire Size Calculator

  1. 1

    Enter your current size

    Width, aspect ratio and rim diameter, as printed on the sidewall.

  2. 2

    Enter the size you are considering

    The comparison updates immediately.

  3. 3

    Check the difference

    Overall diameter, circumference and ground clearance change.

  4. 4

    Read the speedometer error

    What your speedometer would show at a true 60 mph or 100 km/h.

Frequently asked questions

In 225/45R17, 225 is the section width in millimetres, 45 is the sidewall height as a percentage of that width, R means radial construction and 17 is the rim diameter in inches.

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