Tire Size Calculator

The tire size calculator computes and compares the physical dimensions of two tires from their standard tire codes: diameter, circumference, sidewall height, and revolutions per mile, alongside the resulting change in speedometer reading.

• Vehicle type. P (Passenger), LT (Light Truck), ST (Special Trailer), or T (Temporary), the letter prefix of a tire code.
• Section width. The three-digit number at the start of a tire code, in millimeters (for example, 215 in P215/65R15).
• Aspect ratio. A whole-number percentage: the sidewall height as a share of the section width (65 means the sidewall is 65 percent of the width).
• Construction. R (Radial) is standard on passenger vehicles; D (Diagonal) and B (Bias-Belted) are less common.
• Wheel diameter. The rim size the tire fits, in inches (15 in P215/65R15).
• Units. Imperial reports inches and miles; metric reports millimeters and kilometers. Settings also holds a step-by-step view of the formulas and intermediate values behind each result.

Section height equals section width multiplied by aspect ratio, divided by 100. Overall diameter equals wheel diameter (converted to the display unit) plus twice the section height. Circumference equals pi times overall diameter. Revolutions per mile, or per kilometer, equals distance divided by circumference. A diameter difference beyond the recommended 3 percent tolerance is flagged, since it affects speedometer accuracy, ground clearance, and drivetrain load.

A P215/65R15 tire has a section height of 215 times 65 divided by 100, or 139.75 mm, about 5.5 inches. Overall diameter is the 15 inch wheel plus twice 5.5, or about 26.0 inches. Circumference is pi times 26.0, about 81.7 inches, giving roughly 776 revolutions per mile (63,360 inches divided by 81.7).

Tire size comparison is essential whenever wheels or tires change on a vehicle. Understanding the dimensional differences between two tire sizes helps prevent mechanical issues, keeps speedometer readings accurate, and maintains safe driving characteristics. Below are practical scenarios where this calculator provides value.

• Wheel Upgrades: A factory P215/65R15 tire compared against a proposed plus-size P225/50R17 checks that the overall diameter stays within the recommended 3 percent difference when upgrading to larger rims.
• Speedometer Calibration: A speedometer reading deviates by roughly the same percentage as the tire circumference change; a 2 percent larger circumference reads about 2 percent low, which matters for both safety and avoiding a speeding ticket.
• Off-Road Preparation: A stock LT265/70R17 compared against a taller LT285/75R17 shows the exact increase in overall diameter and sidewall height, the trade a truck or SUV makes for better ground clearance.
• Fuel Efficiency Analysis: A change in tire size changes revolutions per mile; fewer revolutions means the engine works slightly differently at highway speeds, which can influence fuel consumption over long distances.
• Tire Replacement: A replacement tire from a different manufacturer, carrying a slightly different code, checks for dimensional compatibility with the original by confirming the diameter difference sits within a safe tolerance.
• Automotive Education: Each component of a tire code, width, aspect ratio, construction, and wheel diameter, contributes its own share to the overall dimensions of a tire, with the step-by-step view showing the math behind each measurement.
• Fleet Management: Standardizing tire sizes across a fleet compares candidate tires against the manufacturer-specified size, confirming that every replacement option falls within acceptable dimensional tolerances for consistent odometer and speedometer accuracy.