A tire's size isn't just about fitting the rim — it shapes your speedometer, gearing and ride height. Understanding the numbers on the sidewall is the first step to choosing a safe, compatible replacement or upgrade.
Reading the sidewall
A size like 225/45R18 means: 225 mm wide, a sidewall 45% of that width tall, and an 18-inch rim. The overall diameter is the rim plus two sidewalls, and it's this diameter that determines how the wheel rolls.
Why diameter matters
Changing the overall diameter changes how far the wheel travels per revolution — which shifts the speedometer, the effective gearing, and the ride height. Too big a change and the car reads the wrong speed and can rub.
The 3% rule
A widely used safe guideline is to keep a replacement within about 3% of the original overall diameter, and to verify clearance and wheel offset before committing. Outside that, speedometer correction and suspension work may be needed.
Frequently asked questions
Diameter = rim diameter + 2 × sidewall height, where sidewall = width × aspect ratio. A 225/45R18 tire has a diameter of 18 in + 2 × (225 × 0.45 mm) ≈ 26 in.
A different overall diameter changes the speedometer, gearing and ride height. Keeping the new tire within 3% of the original diameter is the usual safe rule.
Larger width or diameter can rub the wheel arches or suspension when the wheels turn or the car bumps. Always check clearance and offset before buying.