The Standard Tire Size Format
Most passenger vehicle tires in North America follow this format: P215/65R17 91H. It looks like a cryptic code, but each segment has a specific meaning. Once you understand the format, you can read any standard tire size at a glance.
You can also use our tire size calculator to compare sizes and calculate overall diameter automatically.
Breaking Down P215/65R17 91H
P (Service Type)
The first letter indicates what type of vehicle the tire is designed for:
- P: Passenger vehicle (cars, minivans, light SUVs)
- LT: Light truck (pickup trucks, heavy-duty SUVs, towing applications)
- ST: Special trailer (boat trailers, utility trailers)
- No letter: Euro-metric tire, common on European-spec vehicles
LT tires have thicker sidewalls and higher load ratings than P tires of the same size. If your vehicle specifies LT tires, do not replace them with P-metric tires.
215 (Section Width)
The three-digit number is the tire's section width in millimetres, measured from sidewall to sidewall at the widest point. A 215 tire is 215 mm wide (about 8.5 inches). Wider tires generally provide more grip but may affect fuel economy and require wider wheels.
65 (Aspect Ratio)
The number after the slash is the aspect ratio: the sidewall height expressed as a percentage of the section width.
A lower aspect ratio means a shorter, stiffer sidewall. Low-profile tires (45 or lower) offer sharper handling response but a harsher ride and more vulnerability to pothole damage. Higher aspect ratios (65 to 80) give a more comfortable ride and better cushioning.
R (Construction Type)
R stands for Radial, which is the construction method used in nearly all modern tires. Radial tires have cord plies that run perpendicular to the direction of travel, giving them better flexibility and heat dissipation than older bias-ply (B) tires.
17 (Rim Diameter)
The number after R is the rim diameter in inches. This must match your wheel exactly. A 17-inch tire only mounts on a 17-inch wheel. If you change wheel size, you typically need to adjust the tire's aspect ratio to maintain the same overall diameter so your speedometer stays accurate.
91 (Load Index)
The load index is a code that corresponds to the maximum weight each tire can safely support when properly inflated. A load index of 91 means each tire can carry up to 615 kg (1,356 lbs). A vehicle with four tires rated 91 has a total load capacity of 2,460 kg (5,424 lbs). Never replace tires with a lower load index than what the manufacturer specifies.
H (Speed Rating)
The speed rating is the maximum sustained speed the tire is designed for under its rated load. Common ratings:
- S: 112 mph (180 km/h) - standard passenger vehicles
- T: 118 mph (190 km/h) - family sedans and vans
- H: 130 mph (210 km/h) - sport sedans and coupes
- V: 149 mph (240 km/h) - performance vehicles
- W: 168 mph (270 km/h) - high-performance sports cars
- Y: 186 mph (300 km/h) - exotic sports cars
Never install a tire with a lower speed rating than the original equipment specification. A higher speed rating is fine and sometimes beneficial on performance vehicles.
Calculating Overall Tire Diameter
Overall diameter matters when changing tire sizes, because it affects your speedometer calibration, odometer accuracy, and ground clearance.
For P215/65R17:
When upsizing or downsizing rims, adjust the aspect ratio to keep the overall diameter within about 3% of the original to avoid speedometer error and potential clearance issues.
Frequently Asked Questions
Can I put a different size tire on my car?
Yes, within limits. The rim diameter must match your wheel exactly. Section width can vary by one size up or down (e.g., 205 or 225 instead of 215) without issue on most vehicles. Aspect ratio can be adjusted to compensate for a rim size change, keeping the overall diameter close to stock. Always check that the new tire clears your wheel wells, suspension components, and does not rub at full lock. Our tire size calculator can compare overall diameters across different sizes.
What does the UTQG rating on my tire mean?
UTQG stands for Uniform Tire Quality Grade, a US government rating system. It gives three numbers: Treadwear (how long the tire lasts relative to a 100-point reference), Traction (A, B, or C for wet-road stopping distance), and Temperature (A, B, or C for heat resistance at speed). A treadwear of 400 means the tire should last about 4 times longer than the reference tire in government testing. Higher treadwear numbers indicate longer-lasting tires.
What is the difference between summer, all-season, and winter tires?
Summer tires use a softer compound optimized for dry and wet grip above 7 degrees Celsius (45F). All-season tires balance year-round usability but compromise on peak summer grip and winter traction. Winter (snow) tires use a specialized rubber compound that stays flexible below 7C and have tread patterns designed to evacuate snow and grip ice. In climates with real winters, dedicated winter tires on a second set of wheels are significantly safer than all-seasons.
How do I know when my tires need to be replaced?
The primary indicator is tread depth. New tires start at about 10/32 inch of tread. Replace them when they reach 2/32 inch, which is the legal minimum in most jurisdictions. An easy check: insert a penny into the tread groove with Lincoln's head pointing down. If you can see the top of his head, the tread is at or below 2/32 inch. Also replace tires that show sidewall cracking, bulges, or are more than 6 to 10 years old regardless of remaining tread, as rubber degrades over time.
Should all four tires be the same size?
On most vehicles, yes. Mismatched sizes affect handling balance, cause uneven wear, and can confuse AWD and stability control systems. Some performance vehicles use a staggered fitment with wider tires at the rear, which is a deliberate design choice. If you have AWD, it is especially important that all four tires are the same size and have similar tread depths, as AWD systems can be damaged by significant diameter differences between tires.