Look at the sidewall of any tire and you'll see a string of numbers like "225/65R17." That's the tire size, encoded in a shorthand that's universal across manufacturers. Once you can read it, comparing tires, buying the right replacement, and upgrading your wheels become much easier. Here's the complete decoder.
The format: 225/65R17 102H
Reading left to right:
- 225 — section width in millimeters
- 65 — aspect ratio (sidewall height as a percentage of width)
- R — radial construction
- 17 — wheel diameter in inches
- 102 — load index
- H — speed rating
Let's break each down.
Section width (225)
The width of the tire from sidewall to sidewall in millimeters. 225 mm ≈ 8.86 inches. Wider tires offer more grip but reduce fuel economy slightly and may produce more road noise.
Typical widths by vehicle type:
- Small car: 175–205 mm
- Mid-size sedan: 205–235 mm
- Large sedan/SUV: 235–265 mm
- Truck/large SUV: 265–305 mm
- Sports car: 245–305 mm
Aspect ratio (65)
The sidewall height as a percentage of the section width. So 225/65 means the sidewall is 65% of 225 mm = 146.25 mm tall (about 5.75 inches).
Low-aspect-ratio tires (40, 45, 50) have short sidewalls. They handle better (less flex) but ride harder and are more prone to wheel damage from potholes.
Higher aspect ratios (60, 65, 70+) provide a softer ride and more pothole protection at the cost of slightly mushier handling.
Construction letter (R)
Almost always "R" for radial — the modern construction method. Older or specialty tires may show "B" (bias-ply). For passenger cars, this is virtually always R.
Wheel diameter (17)
The wheel this tire fits, in inches. A 17-inch tire goes on a 17-inch wheel. This is the most important compatibility number — no, you cannot put a 17-inch tire on an 18-inch wheel.
Load index (102)
A coded maximum weight each tire can carry at maximum inflation. You must look up the number in a load-index chart:
- 91 = 1,356 lbs
- 95 = 1,521 lbs
- 100 = 1,764 lbs
- 105 = 2,039 lbs
- 110 = 2,337 lbs
Multiply by 4 to get the total load capacity for a set of tires. The replacement tire must meet or exceed the original equipment load index — never go below.
Speed rating (H)
Letter code for the maximum safe speed:
- Q = 99 mph
- T = 118 mph
- H = 130 mph
- V = 149 mph
- W = 168 mph
- Y = 186 mph
For daily driving, H or V is plenty. Pick a tire rated for your vehicle's top speed or higher.
Calculating overall tire diameter
The total diameter of the tire (for speedometer calculations, clearance checks, and wheel-size upgrades) is:
Total diameter = wheel diameter + 2 × sidewall height
For 225/65R17:
- Sidewall height: 225 × 0.65 = 146.25 mm = 5.76 inches
- Total diameter: 17 + 2 × 5.76 = 28.52 inches
- Total circumference: 28.52 × π ≈ 89.59 inches ≈ 7.47 feet
This number matters when you change tire sizes — a larger overall diameter makes your speedometer read low, changes your final drive ratio, and may rub the fender on full turns.
Plus-sizing: wheels go up, sidewalls go down
"Plus 1" means going from a 17-inch wheel to 18-inch while keeping the same overall diameter — by reducing the aspect ratio.
Example: 225/65R17 becomes 225/55R18. Both have roughly the same outside diameter (28.52 vs 27.74 inches — close enough for speedometer accuracy).
Replacement rules
- Wheel diameter must match (unless changing wheels entirely).
- Overall diameter should be within ±3% of original.
- Load index must meet or exceed original.
- Speed rating should meet or exceed original.
- Width should fit the wheel (each wheel specifies a range of acceptable tire widths).
Seasonal markers
"M+S" stands for mud and snow — an all-season designation. A "3-peak mountain snowflake" symbol means true winter-rated performance. Summer tires have neither marking.
Special designations
- XL = Extra Load (higher load index)
- RF = Run-flat (can drive short distances with zero pressure)
- HL = High Load (newer European standard)
- LT = Light Truck (higher load capacity, more stable construction)
Convert with confidence
Our tire size calculator decodes any tire size string, computes overall diameter, and compares two sizes to show the speedometer error. Use it when shopping for tires, planning a wheel upgrade, or just confirming your tires are what you think. Tire sizing is simple once you can read it — and this tool makes the math trivial.