Tire Size Table
| Code part | Example | What it means |
|---|---|---|
| P | P225/45R17 91W | Passenger car tire (LT = light truck) |
| 225 | P225/45R17 | Section width: 225 mm, sidewall to sidewall |
| 45 | P225/45R17 | Aspect ratio: sidewall height is 45% of width (101.3 mm here) |
| R | P225/45R17 | Radial construction (D = diagonal bias, older cars) |
| 17 | P225/45R17 | Wheel/rim diameter: 17 inches |
| 91 | P225/45R17 91W | Load index: 615 kg max load per tire |
| W | P225/45R17 91W | Speed rating: 168 mph (270 km/h) max |
| Size | Width | Sidewall | Overall diameter |
|---|---|---|---|
| 175/65R14 | 175 mm | 113.8 mm | 23.0 in (583 mm) |
| 185/65R15 | 185 mm | 120.2 mm | 24.5 in (622 mm) |
| 195/65R15 | 195 mm | 126.8 mm | 25.0 in (634 mm) |
| 205/55R16 | 205 mm | 112.8 mm | 24.9 in (632 mm) |
| 205/60R16 | 205 mm | 123.0 mm | 25.7 in (652 mm) |
| 215/55R17 | 215 mm | 118.2 mm | 26.3 in (668 mm) |
| 225/40R18 | 225 mm | 90.0 mm | 25.1 in (637 mm) |
| 225/45R17 | 225 mm | 101.2 mm | 25.0 in (634 mm) |
| 225/65R17 | 225 mm | 146.2 mm | 28.5 in (724 mm) |
| 235/75R15 | 235 mm | 176.2 mm | 28.9 in (733 mm) |
| 245/40R19 | 245 mm | 98.0 mm | 26.7 in (679 mm) |
| 245/45R18 | 245 mm | 110.2 mm | 26.7 in (678 mm) |
| 255/35R19 | 255 mm | 89.2 mm | 26.0 in (661 mm) |
| 265/70R16 | 265 mm | 185.5 mm | 30.6 in (777 mm) |
| 275/55R20 | 275 mm | 151.2 mm | 31.9 in (810 mm) |
| 285/35R20 | 285 mm | 99.8 mm | 27.9 in (708 mm) |
Sidewall terms follow the official tire labeling explained by NHTSA, Tires — before buying a different size, run the diameter change through our tire size comparison and check load fits on tire load index & speed rating.
Every sidewall carries the same code and most spec sheets assume you can read it: P225/45R17 91W is width, height, wheel, load and speed packed into eleven characters. The decoder table above splits each part out, and the size table below gives the overall diameter for the sixteen sizes most drivers actually shop for.
The two numbers that trip people up are the middle pair. Width (225) is millimeters from sidewall to sidewall. Aspect ratio (45) is a percentage, not a measurement: the sidewall height is 45% of 225 mm, or 101.3 mm. So a 225/45 tire sits low and firm, while a 225/65 tire of the same width stands 146 mm tall and rides far softer.
Bottom line: overall diameter is the rim plus two sidewalls. A 225/45R17 measures 25.0 inches across, a 225/65R17 measures 28.5 - six centimeters of radius that changes speedometer readings, gearing and clearance. Stay within about 3% of the factory diameter and the numbers on your dash stay honest.
How to use
- Find your code: it is molded into the sidewall of every tire, and the factory size is also printed on a sticker in the driver's door jamb (some cars repeat it on the fuel door or in the fuel filler area).
- Read it in pairs - width and aspect ratio describe the tire, rim and construction describe the fit, and the service description (91W) describes what it can carry and how fast. The decoder row above matches each fragment to its meaning.
- Before buying a different size, check diameter: keep the new size within roughly 3% of the factory number, then verify the exact speedometer error with our tire size comparison calculator.
Frequently asked questions
What do the numbers on my tire mean?
Take P225/45R17 91W apart: P = passenger car (LT means light truck), 225 = width in millimeters, 45 = aspect ratio so the sidewall is 45% of 225 = 101.3 mm, R = radial construction, 17 = wheel diameter in inches, 91 = load index (615 kg per tire), W = speed rating (168 mph). The table above shows the same breakdown row by row.
How is overall diameter calculated?
Diameter = rim + 2 x (width x aspect ratio / 100), with the sidewall in millimeters divided by 25.4 to get inches. Example: 205/55R16 has a 112.8 mm sidewall, so 16 + 2 x 112.8 / 25.4 = 24.9 inches - exactly the value in the table. The math is pure geometry, which is why the table never changes.
Can I put a different size tire on my car?
Within limits. The widely used fitment guide says stay within about 3% of the factory overall diameter so the speedometer, odometer and ABS references stay close to calibrated. The rim diameter can move (17 to 18, say) as long as the sidewall shrinks to compensate - a plus-one fitment. Wider is not automatically better: width beyond the rim's measured range or the wheel well causes rub and hydroplaning risk.
Where do I find my factory tire size?
Three places, in order of authority: the placard on the driver's door jamb, your owner's manual, and the sidewalls of the tires currently on the car. Trust the door jamb over the current tires - a previous owner may have fitted something different, and the placard also lists the pressure the size was chosen for.