Tire Size Comparison

Enter two tire sizes to see how they compare in diameter, circumference, and speedometer impact.

Tire 1: Width (mm)

Tire 1: Aspect ratio

Tire 1: Rim diameter (in)

Tire 2: Width (mm)

Tire 2: Aspect ratio

Tire 2: Rim diameter (in)

Tire 1 diameter —

Tire 2 diameter —

Diameter difference —

Speedometer error (Tire 2 vs 1) —

Revolutions per mile —

Tire diameter = (2 × width × aspect/100 / 25.4) + rim diameter. Speedometer reads based on original tire revolutions. Larger tires make the speedometer read slower than actual speed.

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Common Tire Sizes

Size	Diameter (in)	Common Vehicle
195/65R15	25.0	Compact sedan
205/55R16	24.9	Midsize sedan
225/45R17	25.0	Sport sedan
235/40R18	25.4	Performance
265/70R17	31.6	Truck/SUV
285/75R16	32.8	Off-road truck

How to Compare Tire Sizes

Tire size is expressed as width/aspect ratio R rim diameter (e.g., 225/45R17). The width is in millimeters, the aspect ratio is the sidewall height as a percentage of width, and the rim diameter is in inches.

When changing tire sizes, keep the overall diameter within 3% of original to avoid speedometer errors, ABS issues, and potential rubbing. This calculator shows exact differences.

Tire size notation (e.g., 225/55R17) encodes: section width (225mm), aspect ratio (55 = sidewall height is 55% of section width), construction type (R = radial), and rim diameter (17 inches). Sidewall height = section width × aspect ratio / 100 = 225 × 0.55 = 123.75mm. Overall diameter = rim diameter + 2 × sidewall: 17 inches = 431.8mm + 2 × 123.75mm = 679.3mm (26.75 inches).

Rolling circumference = π × overall diameter = π × 679.3mm ≈ 2134mm (83.9 inches). This drives speedometer accuracy — the ECU calculates vehicle speed from wheel rotations, so changing tire diameter changes displayed speed. A tire 5% larger than OEM shows 5% less speed than actual on the odometer.

Plus sizing: increasing rim diameter by 1 inch while decreasing aspect ratio to maintain the same overall diameter preserves speedometer accuracy. A 225/55R17 (OD 679.3mm) can be replaced with 225/50R18 (OD = 18×25.4 + 2×225×0.50 = 457.2 + 225 = 682.2mm) — within 0.4% of original.

Load capacity and speed rating are printed separately from the size. Never install a tire with a lower load index or speed rating than the vehicle's OEM specification. The tire placard (inside driver's door jamb) lists the required load index and inflation pressure.