Walk into a tire shop and you'll see three different ways of writing tire sizes on the shelf: P-metric (like P225/65R17), LT-metric (like LT265/75R16), and flotation (like 33x12.50R15). They all describe a tire, but they describe different kinds of tires for different jobs.
P-metric — passenger tires
The most common sizing system in the U.S. The P stands for 'passenger.' Format: P[width]/[aspect]R[diameter].
Example: P225/65R17 means 225mm wide, 65% aspect ratio (sidewall is 65% of width), radial construction, 17-inch wheel.
P-metric tires are engineered for cars, crossovers, and light SUVs that spend most of their time on pavement. The load ratings are lower than LT-metric.
LT-metric — light truck tires
LT stands for 'light truck.' Same width/aspect/diameter format as P-metric, but with LT in front. Higher load ratings, stiffer sidewalls, designed for trucks and SUVs that haul or tow.
Example: LT265/75R16. The numbers mean the same things, but the construction underneath is different — usually 10-ply equivalent rated, where a P-metric tire of the same dimensions might be 4-ply equivalent.
Flotation sizing — off-road and light truck
Used mainly for off-road and oversized truck tires. Format: [diameter]x[width]R[wheel].
Example: 33x12.50R15 means 33 inches tall, 12.5 inches wide, 15-inch wheel. The diameter is the actual tire diameter (not derived from aspect ratio). The width is in inches, not millimeters.
Flotation sizing exists because trucks need really tall, really wide tires for off-roading, and the P-metric format gets cumbersome at those sizes.
Euro-metric — the most subtle distinction
Some tires use the P-metric format but without the P prefix (like 225/65R17, no P). These are 'Euro-metric.' Functionally similar to P-metric but with slightly different load calculations — about 2% higher load capacity at the same pressure. For most drivers, they're interchangeable.
What happens if you mix them
Putting a P-metric tire on a 3/4-ton work truck overloads the tire — the sidewalls aren't built for the weight. You can blow a sidewall under a heavy load. Conversely, putting LT tires on a passenger car gives a harsh ride and unnecessary cost.
Always match the type your vehicle's manufacturer specifies. The door-jamb sticker lists it.
The letters in front of the size aren't decorative. They define what the tire can do.