Tire Size Decoder: P215/65R16 Explained
That string of letters and numbers on your sidewall tells you everything about how the tire fits, what it's rated to carry, and how fast it's safe to drive. Here's every character broken down, plus rules for swapping sizes without screwing up your speedometer or load capacity.
Example Tire Size, Piece by Piece
Using P215/65R16 95H as the example -- a very common passenger car tire.
| Character(s) | Meaning | What It Tells You |
|---|---|---|
| P | Service type: Passenger | Built for passenger cars, CUVs, and light SUVs |
| 215 | Section width in millimeters | Tread is roughly 215 mm (8.46 in) wide when mounted |
| 65 | Aspect ratio (sidewall = 65% of width) | Sidewall height = 215 x 0.65 = 139.75 mm |
| R | Radial construction | Belts run radially (across the tire); the modern standard |
| 16 | Rim diameter in inches | Fits a 16-inch wheel |
| 95 | Load index | 95 = 1,521 lb max per tire at rated pressure |
| H | Speed rating | Safe for sustained operation up to 130 mph |
Service Type (The First Letter)
The prefix identifies what the tire is engineered for. If there's no letter, the tire follows Euro-metric sizing -- common on European vehicles.
| Prefix | Type | Usage |
|---|---|---|
| P | P-metric (Passenger) | Most passenger cars, minivans, and light CUVs |
| LT | Light Truck | Pickup trucks, full-size SUVs, commercial vans; higher load ratings |
| ST | Special Trailer | Boat trailers, utility trailers, campers; not for vehicles |
| T | Temporary spare | Donut / space-saver spare; limited speed and distance |
| (none) | Euro-metric | Similar to P-metric but slightly different load calculation |
P vs LT on a truck: Do not replace LT tires with P-metric if you haul or tow heavy loads. P-metric tires in the same size have lower weight carrying capacity (about 10% less on many trucks) and may fail under load.
Aspect Ratio (The Number After the Slash)
Aspect ratio is the ratio of sidewall height to section width, as a percentage. Lower number = shorter sidewall = stiffer handling but a rougher ride.
| Aspect Ratio | Sidewall Feel | Typical Use |
|---|---|---|
| 70 - 75 | Tall, cushioned ride | Trucks, older sedans, LT applications |
| 60 - 65 | Balanced comfort and handling | Family sedans, crossovers, most economy cars |
| 50 - 55 | Sportier feel; quicker response | Sport sedans, coupes, sporty trim levels |
| 40 - 45 | Stiff; minimal sidewall flex | Performance cars, sport sedans with large wheels |
| 25 - 35 | Very stiff; ride suffers on rough roads | High-performance, track-focused, 19"+ wheels |
Construction (R, B, or D)
The letter between aspect ratio and rim diameter tells you how the internal belts are oriented.
| Letter | Construction | Notes |
|---|---|---|
| R | Radial | Belts run across (radially) at 90 deg to the tread; ~99% of tires today |
| B | Bias belted | Rare today; still used for some trailers and classics |
| D | Diagonal bias | Found on older trailers, motorcycles, and vintage cars |
| ZR | Radial, Z-speed | Speed rated over 149 mph; original Z-rating spec |
Load Index (The Two- or Three-Digit Number)
The load index tells you the maximum weight each tire can support at its rated air pressure. It appears just after the rim diameter. Never install a tire with a lower load index than the vehicle's placard calls for. The table below is the industry standard mapping published by the tire and rim organisations, not a federal one — see the fact check under it for what the regulations do and do not say.
| Load Index | Max Load per Tire | Typical Vehicles |
|---|---|---|
| 82 | 1,047 lb | Subcompacts |
| 88 | 1,235 lb | Compact sedans |
| 91 | 1,356 lb | Midsize sedans |
| 94 | 1,477 lb | Sedans, small crossovers |
| 95 | 1,521 lb | Midsize crossovers |
| 97 | 1,609 lb | Crossovers, minivans |
| 100 | 1,764 lb | Larger CUVs, minivans |
| 103 | 1,929 lb | SUVs, light trucks |
| 108 | 2,205 lb | Full-size SUVs, half-ton trucks |
| 112 | 2,469 lb | Full-size trucks |
| 121 | 3,197 lb | 3/4-ton trucks, heavy SUVs |
Speed Rating (The Last Letter)
The speed rating is the maximum sustained speed the tire is rated for at full load, under the tire industry’s own test standards rather than a federal certification. Replacing with a lower rating can void warranties and is unsafe.
| Rating | Max Speed | Typical Application |
|---|---|---|
| Q | 99 mph (160 km/h) | Winter / studded tires |
| S | 112 mph (180 km/h) | Family sedans, minivans |
| T | 118 mph (190 km/h) | Family sedans, light trucks |
| H | 130 mph (210 km/h) | Sport sedans, performance crossovers |
| V | 149 mph (240 km/h) | Sport / performance cars |
| W | 168 mph (270 km/h) | Exotic / sport tires |
| Y | 186 mph (300 km/h) | High-performance sport cars |
| (Y) | Over 186 mph | Supercars, track-focused vehicles |
Metric to Imperial: Converting Tire Size to Inches
Want to know your tire's overall diameter? Use this formula:
Overall diameter (in) = rim diameter + (2 x sidewall height in inches)
Sidewall height (in) = section width (mm) x aspect ratio / 100 / 25.4
For P215/65R16: sidewall = 215 x 0.65 / 25.4 = 5.50 in. Overall = 16 + (2 x 5.50) = 27.0 in.
Some off-road tires use the alternative format like 33x12.50R17 where the numbers are already in inches: 33-inch overall diameter, 12.5-inch section width, 17-inch rim.
Rules for Swapping Tire Sizes
You can often change tire sizes, but you have to keep the overall diameter close to stock or the speedometer, odometer, ABS, stability control, and transmission shift points will be wrong.
- Stay within 3% of stock diameter. Most sources agree anything beyond that throws off speedometer and safety systems.
- Plus sizing: go up an inch in rim size and drop 10 points of aspect ratio to keep diameter roughly equal (e.g., 215/65R16 -> 225/55R17).
- Load index must match or exceed the vehicle door jamb placard.
- Speed rating must match or exceed the OEM rating -- especially on performance vehicles.
- Width affects load and fitment: wider tires may rub suspension or fenders on full steering lock.
- All four tires on AWD/4WD should match brand, size, and ideally tread depth to avoid drivetrain damage.
Extra Sidewall Markings You'll See
| Marking | What It Means |
|---|---|
| M+S | Mud + snow; all-season rated |
| 3PMSF (three-peak mountain snowflake) | Meets severe snow service standard |
| DOT XXXX XXXX WWYY | DOT code; last 4 digits = week and year of manufacture (e.g., 2923 = week 29 of 2023) |
| Treadwear / Traction / Temperature (UTQG) | Treadwear rating (higher = longer life), Traction grade (AA-C), Temperature grade (A-C) |
| Max Load / Max Press | Max load at max cold inflation pressure (different from door jamb placard) |
| TPC Spec Number | GM-specific tire performance criteria code |
| XL or Reinforced | Extra Load rating; higher load capacity at higher pressure |
What the regulations actually say about these two markings
Load index and speed rating are the two sidewall markings people most often assume are government safety ratings. They are not, and knowing what governs instead changes how you check a replacement tire.
-
Commonly claimedLoad index and speed rating are federal safety ratings.
VerifiedNeither appears in the federal tire standard. FMVSS 139 contains no speed symbol, no load index and no service description anywhere in its text. The one reference in the regulations is FMVSS 110 S4.3(i), which permits “the load range identification symbol, load index, and speed rating” on the vehicle placard “at the manufacturer’s option” — optional, and not defined there. The tables come from the industry bodies FMVSS 139 S4.1.1(b) names: the Tire and Rim Association, the European Tyre and Rim Technical Organization, JATMA and others.
-
Commonly claimedYou need a load index table to find out what your tire can carry.
VerifiedThe tire already tells you, in plain units. FMVSS 139 S5.5(d) requires “the maximum load rating” to be marked on each sidewall, alongside the maximum permissible inflation pressure. The index is a shorthand for a number that is moulded onto the tire in kilograms and pounds — read that instead of looking up the code.
-
Commonly claimedMatch or beat the placard’s load index and the tire is adequate.
VerifiedThe governing rule is the axle, and there is a derating step most people never hear about. FMVSS 110 S4.2.2.1 requires that “the sum of the maximum load ratings of the tires fitted to an axle shall not be less than the GAWR of the axle system” on the certification label. And S4.2.2.2: when passenger-car tires are installed on a multipurpose passenger vehicle, truck, bus or trailer, “each tire’s load rating is reduced by dividing it by 1.10”. A P-metric tire on a truck, SUV or trailer legally carries about nine percent less than its sidewall figure.
Sources
Documents this page was checked against. How we verify.
- SAE-adjacent industry standards via 49 CFR 571.139 S4.1.1(b)FMVSS 139 accepts tire size designations listed in publications of the Tire and Rim Association, the European Tyre and Rim Technical Organization, the Japan Automobile Tire Manufacturers' Association, the Tyre & Rim Association of Australia, and others. The load-index and speed-rating tables below are those organisations' figures; there is no federal equivalent. FMVSS 139 also sets what the tire itself must be marked with, including the maximum load rating on each sidewall (S5.5(d)).
- 49 CFR 571.110FMVSS 110, Tire selection and rims. S4.2.2.1 requires the sum of the maximum load ratings on an axle to be at least the axle GAWR from the certification label; S4.2.2.2 divides a passenger-car tire's load rating by 1.10 when it is fitted to an MPV, truck, bus or trailer; S4.3(i) is the only place the CFR mentions load index and speed rating, and only as an optional placard marking.
Related Quick References
Sources
Documents this page was checked against. How we verify.