Ever wonder what the numbers on a tire mean? The typical tire size is expressed something like this: P245/45R17 93H
M+S. Those numbers and letters can seem like Greek, so let's figure out how to read tire sizes by defining
them one at a time.
P: The first letter stands for
the tire's intended use. P is the most common and it means Passenger.
LT stands for Light Truck. An LT tire has a sturdier construction than
a P tire, and it can carry a greater load, which we will talk about
later.
245: The width of the tire,
from sidewall to sidewall, in millimeters. A 245 is 245 millimeters, or
24.5 centimeters, wide. The tread width is actually somewhat smaller
than the tire's actual width.
45: Aspect Ratio. This is the
sidewall's height from the inside diameter to the outside diameter. It
is expressed as a percentage of the width. A 245/45 is 45 percent as
tall as it is wide, making it approximately 110.25 millimeters tall. As
aspect ratios decrease, the tire's firmness increases. Smaller aspect
ratios, such as 35 or 40 are generally reserved for performance tires
on sports cars.
R: The tire's type of
construction. R stands for Radial, and all new cars and light trucks
today use radials. You may also see B in this spot for Bias Ply. Bias
ply tires were used prior to the mid 1970s.
17: The tire's inner diameter,
which matches the outer diameter of your car's wheels.
93: Load Rating. Load ratings
range from 0 to 279 and each has a corresponding weight associated with
it. A 93 load rating means that tire is certified to carry up to 1433
pounds at maximum inflation pressure. If a car weighs 4000 pounds, then
each tire handles 1000 pounds with no passengers or cargo aboard. If
you intend to tow a heavy boat or trailer, you should buy tires with a
higher load rating. This will require some math to figure out the
vehicle weight, the trailer weight, and the weight of a load you may
carry inside the vehicle while towing. Add it all up, divide by four,
and make sure your replacement tires have a corresponding load rating.
H: Speed Rating. All passenger
tires have a speed rating expressed as a letter. Those letters and the
corresponding speeds the tires are capable of are included in this
chart: S: 112 mph T: 118 mph U: 124 mph H: 130 mph V: 149 mph W: 168 mph Y: 186 mph Z: over 186 mph
A Speed Rating is really an indication of a tire's ability to dissipate
heat to avoid a blowout. Higher speeds mean greater heat buildup. Tires
with higher speed ratings are constructed to handle heat better. In
general, they also ride harder than tires with lower speed ratings. S-,
T-, and U-rated tires are considered regular passenger-car tires. H-
and V-ratings are reserved for touring tires, which are generally found
on sport sedans. The Z rating is for performance tires found on sports
cars.
M+S: Mud and Snow. This is an
all-season tire. A summer performance tire would have no designation
here.
No you will never need to ask the tire guy: "What do the number on tires mean?" Instead refer to this neat little guide on How to Read Tires!
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Ever wonder what the numbers on a tire mean? The typical tire size is expressed something like this: P245/45R17 93H
M+S. Those numbers and letters can seem like Greek, so let's figure out how to read tire sizes by defining
them one at a time.