The numbers on your tire sidewall might look like a secret code, but they actually contain critical information about your tire's dimensions, load capacity, and speed rating. Understanding these numbers helps you buy the right replacement tires and compare sizes accurately.
Decoding a Tire Size: P225/65R17 102H
Let's break down each part:
P — Tire Type - **P** = Passenger vehicle tire - **LT** = Light truck tire - **ST** = Special trailer tire - No letter = European metric tire (functionally similar to P-metric)
225 — Section Width The width of the tire from sidewall to sidewall, measured in millimeters. In this case, the tire is 225mm (about 8.9 inches) wide.
65 — Aspect Ratio The height of the sidewall as a percentage of the width. An aspect ratio of 65 means the sidewall height is 65% of 225mm = 146.25mm. Lower numbers mean a shorter sidewall (sportier look), higher numbers mean a taller sidewall (more comfortable ride).
R — Construction - **R** = Radial construction (99% of modern tires) - **B** = Bias-ply (mostly for trailers and specialty vehicles) - **D** = Diagonal bias construction
17 — Wheel Diameter The diameter of the wheel the tire fits, measured in inches. This tire fits a 17-inch wheel.
102 — Load Index The maximum weight the tire can support when properly inflated. 102 = 1,874 lbs per tire. Look up load index charts for specific values.
H — Speed Rating The maximum speed the tire is designed for: - S = 112 mph | T = 118 mph | H = 130 mph - V = 149 mph | W = 168 mph | Y = 186 mph
How to Calculate Overall Diameter
The overall diameter determines your speedometer accuracy and ground clearance:
Diameter = (2 × Sidewall Height) + Wheel Diameter
Using our example (225/65R17): 1. Sidewall height = 225 × 0.65 = 146.25mm 2. Convert to inches = 146.25 ÷ 25.4 = 5.76 inches 3. Overall diameter = (2 × 5.76) + 17 = 28.52 inches
Or skip the math and use our Tire Diameter Calculator.
Comparing Tire Sizes
When upgrading or changing tire sizes, you want to stay within 3% of your original overall diameter to maintain speedometer accuracy. Use our Tire Size Calculator to compare any two tire sizes side by side and see the exact differences.
Common Tire Size Questions
Can I put a wider tire on my car? Yes, within limits. Going 10–20mm wider is usually fine. Going too wide can cause rubbing on turns.
What happens if I change tire diameter? Your speedometer will be incorrect. Use our Speedometer Correction Calculator to find out by how much.
Do bigger tires affect fuel economy? Larger, heavier tires generally reduce fuel economy slightly. Read our guide on tire upgrades for fuel efficiency.