Air, Speed, & Load: Tire stats roll

Breaking the code on your tire’s sidewall

(Don’t let it blow up in my face.)

Working the quick lube seemed easy enough–change the oil; check the fluid levels, lights, and wipers; and check and adjust tire pressure. However, sometimes, I’d read the tire gauge and then feel the need to immediately shield myself from the rubber balloon. It was stretched taut, hard-pressed to contain the pressure; I felt like it could pop at any minute. It had to be done. I’d scrunch up my face and turn away as I stretched out an arm to deflate the tire. Maybe a little dramatic but tires do have a maximum pressure that should not be exceeded. Whereas it is posted on the side of the tire leads some to believe this is the recommended inflation for the tire. Nope. The best place to learn proper tire inflation is completely prejudiced. It is determined by the vehicle the tires are supporting and will not be found on the tire itself. Tires can go on all sorts of vehicles not just yours. The vehicle manufacturer has determined the best air pressure to gain traction and rolling performance for your vehicle and plastered it in plain sight for the driver (unless the vehicle’s been in a collision and re-painted and the sticker not replaced.). Much more information can be ascertained from both sticker and tire and best operation goes to a matching set.

Those driving around with excessive pressure in their tires will experience a very bumpy ride as there is no cushion and every little gouge in the road is transferred directly to your seat (perhaps a little might be absorbed by the vehicle’s suspension.)

If you open up the driver’s door and visually skim along the door jamb–sometimes on the door itself where it meets the jamb–you will find a label very similar to this:

IMG_0956 - Copy
Pay attention to the numbers in the red-outlined box–I will be deciphering them.

Cold tire pressure tells you what your tire gauge should read first thing in the morning before you drive your vehicle. You are welcome to add two-to-three psi to this number if you have driven and the tire has heated up. As air particles warm, heat is energy and the particles transfer it into motion, increasing pressure. Thus, the tire will read a higher pressure. However, please keep it reasonably under this number:

IMG_0965 (2)

Your tire may not have a limit of 44 psi, but as you are checking tire pressure, you will find the exact number on the sidewall. The type/font looks big in this photo. However, it make take some “Where’s Waldo?” to finally snag the data. Your tires may look under-inflated because they sag a little in the sidewall under the weight of your car, but that is not the case. Tires need this give in order to perform as intended.

IMG_0961 (2)
Focus on 94T: We don’t want your tire flying apart or unable to hold up under the weight.

I could breakdown the details of tire size but prefer to avoid that topic because the details involve calculations like ratios and I’d rather not go through the research and, as for most drivers, I would like to guess you would have installed what is recommended by the car’s maker in order for your vehicle to perform at its safest capacity. Most repair shops and tire stores can look-up what size is recommended for your make and model without glancing at the vehicle itself. However, I inserted this picture for the numbers succeeding the size: 94T … and letter, I might add. You will note that these are included on the tire label at the beginning of this article and, oops, it looks like the car dealer did not care to concern themselves with speed rating when replacing the tires on this vehicle. The letter ‘T’ following the ’94’ does not match the label. (I will cover the ’94’ which is the load index next.)

If you normally keep it under 90 mph or so, speed rating is of little concern since I have rarely seen tires installed that were rated lower, but awareness, I believe, is best. Speed rating is the letter following the number. The following information was found from a search on tirerack.com.

“The most common tire speed rating symbols, maximum speeds and typical applications are shown below:

L 75 mph 120 km/h Off-Road & Light Truck Tires
M 81 mph 130 km/h Temporary Spare Tires
N 87 mph 140km/h
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

When Z-speed rated tires were first introduced, they were thought to reflect the highest tire speed rating that would ever be required, in excess of 240 km/h or 149 mph. While Z-speed rated tires are capable of speeds in excess of 149 mph, how far above 149 mph was not identified. That ultimately caused the automotive industry to add W- and Y-speed ratings to identify the tires that meet the needs of new vehicles that have extremely high top-speed capabilities.

W 168 mph 270 km/h Exotic Sports Cars
Y 186 mph 300 km/h Exotic Sports Cars

While a Z-speed rating still often appears in the tire size designation of these tires, such as 225/50ZR16 91W, the Z in the size signifies a maximum speed capability in excess of 149 mph, 240 km/h; the W in the service description indicates the tire’s 168 mph, 270 km/h maximum speed.”

As you can see, the vehicle manufacturer of my example car suggests installing tires that can remain intact undergoing speeds up to 149 mph. However, the tires that were installed can only withstand speeds of 118 mph. Based on this driver’s habits, the latter rating is well enough protection for their daily driving habits.

As for load index, which is the ’94’ stamped on the tire and exceeds the recommended ’93’ on the sticker. Load index indicates how much weight the tire was meant to support when it was made. Each tire can be assumed to support one-quarter of the vehicle’s total weight (unless you have two pairs on the rear like a heavy-duty truck).

Again, as quoted from tirerack.com, here is the decoding chart for load index:

Load Index Pounds Kilograms Load Index Pounds Kilograms
71 761 345 91 1356 615
72 783 355 92 1389 630
73 805 365 93 1433 650
74 827 375 94 1477 670
75 853 387 95 1521 690
76 882 400 96 1565 710
77 908 412 97 1609 730
78 937 425 98 1653 750
79 963 437 99 1709 775
80 992 450 100 1764 800
81 1019 462 101 1819 825
82 1047 475 102 1874 850
83 1074 487 103 1929 875
84 1102 500 104 1984 900
85 1135 515 105 2039 925
86 1168 530 106 2094 950
87 1201 545 107 2149 975
88 1235 560 108 2205 1000
89 1279 580 109 2271 1030
90 1323 600 110 2337 1060

My example vehicle is over-supported as the tires can withstand more weight than the vehicle manufacturer expected, so the driver is well-covered.

If you’re looking at the sidewall there is one long number I have not covered yet and it is preceded by DOT. This Department of Transportation number is used for tracking purposes as all I have ever need use of it is to record the number when replacing tires on a vehicle in the shop.

I haven’t covered all the numbers related to tire-shopping, but I wanted to ‘clear the air’ on some of the lesser celebrated specifics as they can make a difference as I have elaborated. Before I close, I should mention the M & S on the sidewall decodes into mud and snow. The other common option is A/S, as in all-season. Feel safer in your knowledge as you drive down the road and know that knowledge can help in negotiating. Please see my suggested links to see the studies confirming this. And, of course, feel free to toss a comment or two regarding anything you feel should be addressed that I have left out.

I will add that the P before tire size is for passenger car. Another common abbreviation is LT for light truck. And T for the spare is for temporary. Please do not drive for extended periods of time on a donut-sized spare. If you have a regular-sized tire for a spare that is fine, but the temporary is meant only to get you to the nearest safe stop (and is commonly discovered under-inflated during oil changes. I advise to specifically request your technician check your spare’s tire pressure at your next service as it can be easily avoided if said technician believes you are in a rush to have your car back and spares are not the easiest tires to reach.).

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