The Numbers Behind Wheels + Tires: Tires Next


     Ok.  This time we’ll take a look at the way tires are measured and presented.  For example:

[255/40/17] [94W] [180 AA A]

     Let’s start with the first set of numbers.  This is the most important set I guess, as this measures the size of the tires: [255/40/17] [94W] [180 AA A].  The set in red represents the sizing of the tire.  Here’s a diagram:


[255/40/17].  The “255” of that set is the Section Width (or tire width.  same thing) in millimeters.  Remember that.  So a tire of size [255/40/17] is 255 millimeters in width.

[255/40/17].  The “17” represents the Tire/Wheel Size this set of tires is for.  Obviously the “17” is in inches as all wheels are measured in inches.

[255/40/17].  The “40” represents the Tire Sidewall, or the “height” of the tire.  We talked about that briefly last time.  That is basically the distance from the edge of the wheel to the edge of the tire when looking from the side of the car.  The “40” is a percentage, not an actual unit.  It’s the percentage of the width of the tire, so our [255/40/17] tire has a sidewall of 40% of 255 millimeters.  That equals to about 102 millimeters.  Now remember, this won’t be the actual measurement from the edge of the tire to the edge of the wheel, as the tire will be compressed at that point.  Remember, since this is a percentage, the sidewall of a 255/40/17 will be different from the sidwall of a 205/40/17.  Obviously, 40% of 255 millimeters is more than 40% of 205 millimeters.  Keep that in mind.

     The second set [255/40/17] [94W] [180 AA A] shows the Load Rating and Speed Rating of the tire.

[94W].  The Load Rating basically tell us how much weight a tire can handle.  We don’t have to worry about that really since most tires are divided between being either for everyday cars, or for trucks and SUVs.

[94W].  But the Speed Rating is a bit more important if you plan on racing or going for the land speed record.  This basically tells us how fast the tire can go.  Remember, the faster we go, obviously the more we stress the tire.  The speed ratings here aren’t 100% accurate, but are a basic guidline.

M 81 mph 130 km/h  
N 87 mph 140km/h Temporary Spare Tires
P 93 mph 150 km/h  
Q 99 mph 160 km/h Studless & Studdable Winter Tires
R 106 mph 170 km/h H.D. Light Truck Tires
S 112 mph 180 km/h Family Sedans & Vans
T 118 mph 190 km/h Family Sedans & Vans
U 124 mph 200 km/h  
H 130 mph 210 km/h Sport Sedans & Coupes
V 149 mph 240 km/h Sport Sedans, Coupes & Sports Cars
W 168 mph 270 km/h Exotic Sports Cars
Y 186 mph 300 km/h Exotic Sports Cars
That’s the basic outline for the speed ratings.

The final set [255/40/17] [94W] [180 AA A] measures the Treadwear Grade, Traction Grade, and Temperature Grade.

[180 AA A] This is the Treadwear Grade.  It measures the maximum life of a tire.  The way they test it is to run a tire for 7,200 miles, and mark that as a “100”, or can last for 100% of the test.  A “180” would last 180% of that test, or 12,960 miles.  And a “400” would last for 28,800 miles.  This is all, of course, not exact.  The way they test it is to run the tire round a 400 mile test loop.  That’s obviously different from the way we use and abuse our tires everyday.  So take it with a grain of salt.  A treadwear grade of about “200” can last for about 10,000 miles, give or take.  Take into account your driving conditions; the weather every year, your driving habits, the road conditions, and obviously the car you’re driving.

[180 AA A] is the Traction Grade.  This test is not accurate for real world driving conditions, and should not be taken very seriously.  The way its tested is on wet concrete, with locked brakes at a constant 40 mph.  Obviously all season tires will do better here, as the tests are done in wet conditions.  Semi-slicks, or semi-racing tires will suffer due to their focus on dry pavement driving.

[180 AA A] is the Temperature Grade.  The ratings are either “A”, “B”, or “C” here.  It determines how well a tire can dissipate heat and withstand a certain speed.  “C” means the tire can operate below 100 mph, while “B” is for under 115 mph, and an “A” means the tire can run over 115 mphs.  This is basically the same as the Speed Rating, but the Speed Rating is a bit more accurate.


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April 2009
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© 2009 Rusi Li

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