Tire Size Comparison Calculator

A tire size code carries three measurements: section width, aspect ratio and rim diameter.
Between them they fix the tire’s overall diameter and circumference, which is what actually determines speedometer accuracy, ground clearance and whether a wheel fits under the arch.

Tire Designation Format: P225/60R16

• 225 = Section width in millimeters
• 60 = Aspect ratio (sidewall height as % of width)
• 16 = Rim diameter in inches

Sidewall Height:

Sidewall (mm) = Section Width × (Aspect Ratio / 100)

Overall Tire Diameter:

Diameter (inches) = (2 × Sidewall mm / 25.4) + Rim Diameter (inches)

Tire Circumference:

Circumference = π × Diameter

Speedometer Error when changing tire sizes:

Actual Speed = Indicated Speed × (New Circumference / Old Circumference)

Worked example, P225/60R16:

• Sidewall = 225 × 0.60 = 135 mm
• Sidewall in inches = 135 / 25.4 = 5.315 in
• Diameter = (2 × 5.315) + 16 = 26.63 inches
• Circumference = π × 26.63 = 83.66 inches, so 6.97 feet per revolution

Fitting a plus-size tire (P235/55R17):

• Sidewall = 235 × 0.55 = 129.25 mm → 5.09 in
• Diameter = (2 × 5.09) + 17 = 27.18 inches, which is 0.55 in taller
• Speedometer at 60 mph indicated: Actual = 60 × (27.18 / 26.63) = 61.2 mph
• Diameter change = 0.55 / 26.63 = +2.1%, inside the rule below

Rule of thumb: keep the total diameter within ±3% of the OEM size. Past that you start affecting speedometer and odometer accuracy, ABS and traction control calibration, and on the taller end, clearance at full steering lock.

Ride height moves by half the diameter change, not all of it, because the extra rubber is split between the top and bottom of the tire. A tire an inch taller lifts the body half an inch.