How to Read Tire Sizes: What Every Number Means
P225/65R17 95H. Looks like gibberish, right? Every part of that string means something specific, and once you crack the code you can shop for replacement tires, compare sizes, and know whether an alternative size will fit your vehicle safely.
Compare Tire Sizes
Enter your current and new tire sizes to see diameter difference, speedometer error, and fitment verdict.
Tire Size CalculatorBreaking Down P225/65R17 95H
P = passenger vehicle (you'll also see LT for Light Truck). 225 = tire width in millimeters, sidewall to sidewall. 65 = aspect ratio, meaning the sidewall height is 65% of the width. Lower number = shorter, sportier sidewall. R = radial construction (virtually all modern tires). 17 = wheel diameter in inches.
95 = load index (max 1,521 lbs per tire). H = speed rating (130 mph -- more than you'll ever need legally). Always match or exceed your car's original load index and speed rating when you buy new tires.
How to Calculate Overall Diameter
Overall diameter is the wheel plus both sidewalls. Here's how the math works: For a 225/65R17: the sidewall height is 225mm x 0.65 = 146.25mm. Convert to inches: 146.25 / 25.4 = 5.76 inches. Double it for both sidewalls: 5.76 x 2 = 11.51 inches. Add the wheel diameter: 11.51 + 17 = 28.51 inches total. This number is what determines your speedometer accuracy, ground clearance, and whether the tire fits in your wheel well.
What the Aspect Ratio Tells You
The aspect ratio is how tall the sidewall is relative to the width. A 65-series tire has a taller sidewall that absorbs bumps well and provides a comfortable ride, common on SUVs and sedans. A 45-series or 40-series tire has a short, stiff sidewall that improves handling responsiveness and gives a sportier look, but transmits more road imperfections to the cabin. A 70-series or higher tire is common on trucks and provides maximum sidewall protection for off-road driving.
Speed Ratings Decoded
Speed ratings tell you the max sustained speed the tire can handle. Common ratings include: S (112 mph), T (118 mph), H (130 mph), V (149 mph), W (168 mph), and Y (186 mph). Your replacement tires should match or exceed the speed rating specified by your vehicle manufacturer. Downgrading the speed rating can compromise tire performance and may affect insurance coverage in some jurisdictions.
Can You Change Tire Sizes?
Yes, but there are limits. The general rule is to stay within 3% of the original overall diameter. Going beyond 3% affects speedometer accuracy (your speed reading will be wrong), odometer accuracy (recorded mileage will be off), and potentially ABS and traction control calibration. The Tire Size Calculator compares any two sizes and shows the exact diameter difference, speedometer correction, and a color-coded fitment verdict.
Plus-Sizing: Bigger Wheels, Same Diameter
Plus-sizing: bigger wheel, thinner tire, same overall diameter. For example, moving from 225/65R17 to 245/50R18 maintains nearly the same overall diameter while giving you a wider contact patch and a more aggressive look. The tradeoff is a firmer ride, increased vulnerability to pothole damage, and higher replacement cost. For road trip planning with your current or new tires, the Fuel Cost Calculator estimates fuel expenses by distance.