There’s a code on the side of every single tire that you’ll run across.  Or, more accurately, there should be a code!  If there’s not your best bet is to move on and find a different source.  Any set of wheels without the required manufacturers’ information likely has more problems than just that.

Let’s make this simple and move left to right, interpreting what the characters mean as we go.

P225/50R16 91S

Most tires you will run across will start with the letter P.  This signifies that it is of the “P-Metric” size and is engineered to fit on most passenger vehicles.  This includes but is not limited to minivans, cars, light duty trucks, and SUVs.  A cheap truck tire will have the same markings as an expensive one.  A cheap minivan tire will as well.  Think of the P as standing for passenger.  All it is really, though, is a standard for sizing the wheel.

If it does not begin with a P (like this 225/50R16 91S), it still means it’s a passenger vehicle tire, but it is sized by the metric system.  Most European cars will have tires of this type.  The two sizing conventions are typically very similar.

The size can also begin with an LT, which means the tire was designed for use with a light truck.  But don’t be fooled by the use of the term “light truck tire”, it’s using the term in relation to tractor-trailers and heavy-duty commercial trucks.  So LT means 3/4 and 1 ton pickups that are capable of carrying 3 to 4 tons of payload.

ST tires (ST225/75R15) are “Special Trailer” use tires that are generally an odd size designed for use with a specific cargo trailer, and should never be used on a passenger vehicle.  If you’re looking for a cheap trailer tire, it will likely have ST as the leading characters.

Of course, T stands for “temporary service” and will likely be included on your spare tire, unless you have a full size that you bought with a set.