The Pressure to Keep Rolling (Part 2)


Expertise Includes:

    • Tractor Trailer Accidents
    • Vehicle Collision Reconstruction
    • Crash Data Retrieval
    • Forensic Mapping Technology
    • Motorcycle Collision Reconstruction

One of the first safety concerns for your vehicle should be your tires. The tires on your vehicle or trailer maintain contact with the roadway and assist you in getting to your destination safely. So how do you know that your tires are proper for your vehicle, are being well-maintained, and beyond the obvious threat of tire failure, why is this important to know?

Placard location ALD 2The first place to start is with the vehicle manufacturer and ensuring that your vehicle has the proper tires installed. The easiest and fastest way to start this quest is with the Tire and Loading Information placard. The first place to look is in the driver’s side door jamb. With the door open, search the rear-most portion of the outer door jamb or the actual outer-most section of the door. These are the typical locations; however, with all the different vehicles and manufacturers out there, it

Most Popular Locations to Find Tire and Loading Information Placards

could be in a different place. Can’t find it? The sticker may have fallen off or been painted over. No worries, go to the owner’s manual and you can locate the same information in there that was displayed on that sticker. Search the internet if all else fails. Regardless of where you find the information, it is important to follow the manufacturer’s details. The sizes of the tire have been determined by engineering design to provide the best performance for that vehicle.   Air pressure has been matched for the same purpose, to make the vehicle perform to it’s safest and most efficient ability.

What exactly does that information mean? This is the manufacturer of the vehicle letting the driver (they are the ones in control regardless of who owns the vehicle) know how the vehicle is engineered and designed in relation to the design and engineering of the tires. Some tires are specifically designed for certain vehicles, but more commonly the design of tires is considered by the vehicle manufacturer in relation to the design of the vehicle. Looking at the sticker below you can see what it is saying about the tires required on the vehicle.

Tire Loading Placard ALD (002)

Tire and Loading Information Placard

In the very first column, it is obviously differentiating between the front tires, the rear tires, and (an often overlooked but very important tire) the spare. Depending on the designed purpose of the vehicle, there may be a difference in the front and rear tires on a vehicle. Trucks designed for carrying a heavy payload may be required to use different air pressures or tire sizes to accomplish their job. Knowing this is important; you wouldn’t want to rotate the tires without making sure you considered it.

The next step is the tire size. This is a complicated area because it’s not simply a single number. On this particular vehicle, the size starts with a “P” for passenger. You may find “LT” for light truck or you may find nothing at all. This indicates the purpose the tire was designed for. The next numbers identify the section width in millimeters, measuring from sidewall to sidewall on an inflated tire that is mounted on a wheel. The (55 or 75) equates to the aspect ratio. In other words, the height expressed as a percentage of the tires section width. In this case the sidewall is 65% of the 205mm. The “R” indicates the tire is a radial and the “20” shows that the tire fits a 20-inch rim. Changing one number can have an impact on the vehicle’s performance and safety. It is also a good time to notice that the spare tire is a completely different size tire and rim. This tire is not designed to completely replace your vehicle’s regular tire and rim, but to sufficiently replace one well enough to get you from the roadside to a tire store.

Tire Speed rating ALD (002)

Load Index with Speed Ratings

Often there will be numbers or letters beyond the ones discussed so far. These are important because, again, they identify the engineering design of the tire. One is for the Load Index, describing what weight the tire can support. This is a numerical code that associates the maximum load for a tire at the speed indicated under the specific service conditions.   Just because the tire has the same Load Index does not necessarily mean the tire will hold the same load.  The same tire on different vehicles and different service conditions dictate the limits too. There are charts for this, but staying with the manufacturer’s recommendation is best. A Speed Rating is identified by alphabetical reference.  As you can see on the sticker pictured, though there is no load index, there is a speed rating. This particular vehicle requires a speed rating “S”. This is charted too and shows the maximum speed a tire can be safely operated at the given load index and under the specific service conditions. For the “S” it is 112 mph.

Over to the “Cold Tire Pressure”, meaning the pressure when the tire has not been driven on. This is relating to the tire, not the weather, though the ambient temperature and atmospheric pressure can influence tire pressure.  What is being measured here is the tire pressure before the vehicle is driven. This sticker indicates that all four tires require the same pressure and the spare a different pressure. This may not be the case for different vehicles so do not assume that the front, rear, and the spare are the same. Further, the rating for the vehicle may not be the same as the maximum cold pressure written on the sidewall of the tire. The manufacturer’s specifications are limited to that particular tire and vehicle/tire combination, and takes into consideration the ride, handling and loading limits as they have designed the vehicle. Keeping watch on the tire pressure is extremely important for the life of your tires and the safety of your ride. A tire that is underinflated tends to overheat from flexing during the ride. This heat causes a breakdown in the integrity of the tire’s internal components and leads to tire failure in what could otherwise be a perfectly good tire. If you are already on your journey before you have the opportunity to check your tire pressure, it is recommended that you apply 4 psi more than what the tire is rated for “cold” so that you can be properly inflated on a “hot” tire.

Tire Pressure Gauge ALD (002)If you can look at a tire and see that it is low on pressure, that tire is extremely low on pressure. A tire may look absolutely fine visually but checking the tire pressure with a pressure gauge is the only way to truly tell.  I once test drove a motorcycle with low tire pressure and it felt like I was driving through thick mud. Which brings up another point, that motorcycle’s tires looked as if they were properly inflated but they were more than 10 psi low. Further, the tires will lose air over time (1 psi per month for motorcycle, passenger and light truck, and 2 psi per month for commercial tires), so it is good practice to check your tire pressure monthly to ensure you are maintaining the proper pressure, enabling you to get the best life of your tires, the best performance out of your vehicle, and all you can do to ensure the safety of your family.

I have only covered a simple review of tire sizes and tire pressure compared to vehicle application in relation to a sticker on my work vehicle; however, you can see how complicated tires can be. Listening to a tire engineer/expert at a conference, he expressed an opinion that roughly 7 % of tire failures were a result of manufacturer defect. That’s not a lot percentage wise, but when you think of all the tires out there, 7% is a large number. There is a chance that 3 tires of the 42 tires and spare tires on the vehicles and trailers on my farm are defective; therefore, it is only sensible to increase safety and lessen my odds of having tire problems by using the right tires and maintaining them properly. I encourage you to do the same, take care of your tires.

Aaron (Al) Duncan II, ACTAR, is a vehicle collision reconstructionist with Warren and President of SCARS. Prior to joining Warren, he worked for 23 years as a South Carolina Highway Patrol Trooper to include 10 years as a Multi-Disciplinary Accident Investigation Team (M.A.I.T.) member. Al is accredited as a Traffic Accident Reconstructionist by The Accreditation Commission for Traffic Accident Reconstruction. He investigated in excess of 1000 vehicle accidents and incidents, as a trooper. Then, as a member of M.A.I.T. for 10 years, he was involved in over 1000 detailed investigations and collision reconstructions. Al has testified multiple times in state courts and he has been court qualified as an expert in accident investigation and collision reconstruction. Al’s work expertise focuses on investigating and reconstructing vehicle collisions involving single and multi-vehicles, animals, pedestrians, motorcycles, heavy trucks, and commercial vehicles. He is also a skilled user of forensic mapping technology and computerized collision diagramming software for collision scene analysis. Al is experienced in the data download and analysis of airbag black boxes (Crash Data Retrieval Units) in automobiles, pickup trucks, and SUVs. He holds a Bachelor of Science Degree in Political Science from Lander University in Greenwood, South Carolina and completed the Law Enforcement Basic Program at the South Carolina Criminal Justice Academy in Columbia, South Carolina.

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