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Tires: Part 1


Pressure, Stopping Distance and Causality - Part I


By: Patrick Sundby, Accident Investigator

Specializing in Low Speed and Catastrophic Crashes



There is a lot of information regarding tires, so much so this information goes way beyond the recommendations and reviews on the megastore website.  Here we will discuss, from a post-collision perspective, some vehicle specifications, standard tire information, and how tire pressure monitoring systems (TPMS) work.  We will then examine how tire pressures relate to automotive collisions in helping to determine causality.


Vehicle Specifications

Vehicles sold in the United States have a placard in the driver’s door jamb or inner door.  This placard contains some information we need to have to explore the tires including the vehicle manufacturer recommended tire size, load rating, and tire pressure.  Here is an example:


(There is a second placard specifically for tires but this should be confirmed against the above placard as the second one doesn’t contain any vehicle identifying information such as a VIN.  In this photograph the last six digits of the VIN have been omitted.)

Tire Size

The majority of modern tires have writing on the sidewall which explains the tires dimensions and other important characteristics.  What does it mean?  In the tag above the tire sizes for front and rear are listed.  The 265 is the width, in millimeters, of the tread face.  The next number, 70, is the height of the tire sidewall as a percentage of the tread face (in this case 70% of the 265).  The “R” makes the tire construction a radial.  Finally, the 17 is rim diameter in inches.

Tire Pressure

Notice the listed tire pressure is supposed to be cold.  Tires have to sit for at least eight hours out of direct sunlight before they are considered cold enough for taking a reading.  Gases expand as they are heated and the minimum cold pressure is set so the tire will be at the optimal pressure once at operating temperature; accordingly, if a tire is at or below the minimum and is at operating temperature, the pressure was even lower when the tire was cold.

Tire Pressure Monitoring System (TPMS) 

The TPMS became a mandated standard after the fallout of the Ford Explorer & Firestone tire event.  The federal government wanted a system which would alert drivers to “low” tire pressure(s).  There are two types of systems.  The first type is called “direct measurement” and it uses a sensor inside each tire which wirelessly relays the pressure.  The second type is called “indirect measurement” and it uses the anti-lock brake system to determine if a tire is spinning faster than the others.  A tire with less air pressure will have a smaller diameter and therefore will spin faster; the anti-lock brake system can calculate this difference.

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