Reading Tire Size
Tire sizes follow the format P215/65R17 93H. “P” means passenger car. “215” is section width in millimeters. “65” is the aspect ratio—sidewall height as a percentage of section width (65% of 215mm = 139.75mm). “R17” means radial construction with a 17-inch rim. “93H” is the load index (650 kg per tire) and speed rating (H = up to 130 mph / 210 km/h).
Calculating Overall Tire Diameter
Diameter = (2 × sidewall height) + rim diameter. Sidewall height = width × aspect ratio ÷ 100. For P215/65R17: sidewall = 215 × 0.65 = 139.75mm ≈ 5.5 inches; overall diameter = (2 × 5.5) + 17 = 28 inches. Changing tire diameter affects speedometer accuracy, odometer readings, and ground clearance.
Plus-Sizing and Speedometer Error
Going up one rim size (e.g., 17” to 18”) with lower-profile tires keeps overall diameter the same—this is plus-sizing. But a 3% larger overall diameter makes your speedometer read 3% low (actual speed exceeds what is displayed). GPS apps show true speed. Recalibrate after any significant tire size change.
Frequently Asked Questions
Will bigger tires affect my speedometer?
Yes. A taller tire travels more distance per rotation, so actual speed exceeds what the speedometer reads. A tire 3% taller than stock means true speed is 3% higher than indicated — at 60 mph indicated, you’re doing 61.8 mph. Most jurisdictions allow ±4% deviation before recalibration is required. GPS navigation apps show true speed independent of the speedometer.
Can I put any tire size on my car?
No. The tire must clear wheel wells, suspension components, and fenders at full steering lock and full suspension compression. As a general guide: stay within ±3% of OEM overall diameter, and ensure section width is compatible with your rim width (rim width should be 75–90% of tire section width). For lifted or lowered vehicles, consult a fitment guide specific to your make, model, and year.