P255/50R16

This means we have a P-metric general passenger type tire with a 255mm section width.  Nothing fancy so far.  Continuing down the line, we see that the profile is 50% of the section width, which puts it at 127.5mm.  Note, the smaller the third term in the string the lower the profile of the sidewall will be.  Thus, if you’re looking for a low profile tire, or slicks as some people call them, you’ll be looking for a very low number to occupy that space.  Especially if they’re a very wide tire.

When looking at a tire for a truck, people generally want a higher third term for a taller tire, where width isn’t so much of a factor.  But due to manufacturing processes and engineering, you’re generally going to find a higher second term as the profile gets higher.

This brings us to the fourth term, which tells us what the internal construction of the tire is.  I get asked all the time if this separates a cheap tire from an expensive tire, and the answer is always no.  The internal construction of the tire has very little to do with price, since most tires in this day and age are R-type, or radials.

What this means is that the tire construction is a series of rubber plies that radiate out from the center of the tire where it fits around the rim.  In other words, imagine that there are 10 layers of rubber from the outside surface of the tread to the inside where the pressurized air is held.  If you were to cut the tire drop top to bottom to look, it would almost look like a horseshoe.

Other constructions that you might see are D for diagonal, or bias-ply, where the plies of rubber are laid at a 90 degree angle to each other, diagonal across the tire as it would travel on a vehicle.  B indicates belted, which is similar to bias-ply construction except it is reinforced with belts under the tread area.

More than 95% of all tires sold today are of radial construction, cheap tire or expensive tire.

Come to think of it, why people have such a hard time explaining what the section width and aspect ratio are continues to elude me.  People in the tire business for years and years know what it is, can articulate what it is, but can’t translate it from tire lingo to “normal speak”.  I see customers leave my shop every day with bewildered looks on their faces because one of my tech just put them through the information wringer.  So I’ll attempt to break it down in basic term that everyone can understand.

Section width is the width of the tire.   There, simple right?  I’m sure most people reading this are asking “what width?” right now, and that’s where people usually get hung up.  So if we’re looking down at a tire that is stand up in a position where it would roll away if you pushed it, such that we’re staring down at the tread, the section width is basically the width of the tread in millimeters.  It goes a little further than that but measuring from the fattest part of the sidewall on one side to the fattest part on the other side, but that’s not so important.

So a LT275/50R16 91S is 275 millimeters wide from sidewall to sidewall if you were to puncture the cheap tire at its widest point with a ruler.

Now, sidewall aspect ratio is not an absolute measurement like section width is.  It is a percentage of the section width, and determines how “big” your tires are from the rim to the tread.  This is also referred to as the tire section height, aspect ratio, or profile.  For an example, our LT275/50R16 91S measures 137.5 millimeters from the tread to the rim, because 275mm x 50% = 137.5mm.

That means the higher the aspect ration the taller your tire will stand.  At the same time, the greater the tire section, the greater your tire will stand.  So both of those figures play into the actual size of your tire.  Remember that, it’s important.

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.