| Aspect | Overall Diameter |
|---|
Finding Overall Tire Diameter
The number printed on a tire, such as 245/45R18, hides its real height. Overall diameter is what matters for gearing, speedometer accuracy, and fitment, and it comes from adding the wheel diameter to twice the sidewall. The sidewall is the width multiplied by the aspect ratio, which is a percentage, not a fixed measurement.
Why the Aspect Ratio Matters
Aspect ratio ties sidewall height to width, so a wider tire at the same aspect has a taller sidewall. Drop the aspect for a lower-profile look and the tire gets shorter unless you go up in wheel size to compensate. That is the basis of plus-sizing, where wheel diameter rises as the aspect falls to keep overall diameter roughly constant.
Diameter Drives Everything Downstream
Once you know overall diameter you can find circumference, revolutions per mile, and engine RPM at speed. A bigger diameter than stock lowers cruising RPM and makes the speedometer optimistic, reading slower than you are really going. Matching diameter to stock, or recalibrating, keeps the speedometer honest after a tire change.
Frequently Asked Questions
Is overall diameter the same as the rolling diameter?
Almost. The loaded rolling diameter is a touch smaller because the tire flexes under the car, so it is the more accurate figure for gearing and speedometer math.
What does the R mean in a tire size?
It stands for radial construction. The numbers around it are width in millimeters, aspect ratio as a percentage, and wheel diameter in inches.
How do I keep my speedometer accurate?
Choose a new tire with the same overall diameter as stock, or have the speedometer recalibrated for the new diameter.