Tire Size Calculator

Tire dimensions are derived from a combination of tread width, profile ratio (aspect ratio), and wheel rim diameter. First, the sidewall height is found by multiplying the section width by the profile ratio: Sidewall Height = Width × Profile Ratio. The overall tire diameter is then calculated by adding twice the sidewall height to the wheel diameter: Diameter = Wheel Diameter + (2 × Sidewall Height). From there, circumference is computed using the formula Circumference = π × Diameter, while revolutions per distance rely on the inverse of the circumference. When comparing a secondary tire, percentage differences in diameter directly scale the vehicle's speedometer reading via the relationship: Actual Speed = Speedometer Reading × (1 + Diameter Difference Percent).

Imagine your vehicle currently runs on a standard 205/55R16 tire, and you want to see how upgrading to a 225/45R17 tire impacts your vehicle. For the original tire, the width is 205 mm, the profile ratio is 0.55, and the wheel diameter is 16 inches. Converting units and applying the formulas yields a sidewall height of 112.75 mm (4.44 inches) and an overall diameter of 24.88 inches. For the new 225/45R17 tire, the width is 225 mm, the profile ratio is 0.45, and the wheel diameter is 17 inches, resulting in a sidewall height of 101.25 mm (3.99 inches) and an overall diameter of 24.97 inches. The diameter difference is minimal at just +0.36%, meaning your speedometer will remain remarkably accurate, reading 60.2 mph when you are actually traveling at 60 mph.

Always aim to keep your overall tire diameter within 3% of your original factory specification to prevent issues with transmission shifting, anti-lock braking systems, and electronic stability control. Keep in mind that lowering your profile ratio for a sportier look will result in a stiffer ride due to shorter sidewalls absorbing less road shock. Finally, double-check your vehicle's load index and speed rating requirements alongside physical dimensions to ensure safe handling under maximum cargo and velocity.