The Real Reason to Check Your Air Pressure

 

By Gary Ruede, Discount Tire   
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Every year, tire industry organizations along with the National Highway Traffic Safety Administration (NHTSA) have a weeklong campaign called "Tire Week".  During this week, and around holidays, we are reminded to check our tire air pressure and overall condition. They mention it is for our safety and for better gas mileage or range. However, they leave out the why's. So, in the best interest of consumers, we will discuss the why's.
Tires are a mess of measurements that consist of inches for wheel diameter and millimeters when referring to tire width. This continues with PSI (pounds per square inch) and BAR (metric). One BAR equals 14.50377 psi. These are the units of measure used by the consumer to set and adjust their tire pressures.

When engineers design tires they use KPA (kilopascal) which is named after the French mathematician-physicist Blaise Pascal. KPA is an International System of Units (SI) which is used by engineers globally, so design results are the same. So why is this relevant?

 

(225/65R17 SL 102)

Above is the conversion from KPA to PSI as it relates to load. 35.7 psi rounds up to 36 and 36.5 psi rounds down to 36 psi which is a difference of 35 lbs. So, one psi can vary by 35 lbs.

The tire load index is determined by the calculated load and rounded to the closest index. The KPA used to calculate load is 250 KPA. Above you will see that this tire carries 1852 at 250 KPA and then rounded to 102 load index (closest index to 1852 lbs.) 

 

LOAD INDEX	LOAD
101	1819
102	1874
103	1929

 

So, the load of a tire can be rounded by 27 lbs., the psi load can vary by 35 lbs. for a total of 62 lbs. and we haven't even talked about setting the air pressure yet.

 

LOAD INDEX
PSI	30	31	32	33	34	35	36
102	1620	1663	1705	1748	1790	1832	1874

2022 ETRTO Q=Qt(p/Pt)a (exponent of .80)

The load inflation table is above. A one PSI change from 36 psi to 35 psi is 42 lbs.
The average tire pencil gauge has +/-2.5 psi accuracy, 105 lbs. difference. So, when setting the air pressure in your tire the actual load carrying capability can vary by 167 lbs.

The average passenger car tire loses 2-3 psi on average per month.  So, after a month your 272 lbs. per tire.  272 X 4 = 1088 lbs.
Tires do not carry the load of your vehicle, pressure does. Pressure dictates the wear and mileage your tires can achieve and contribute greatly to mileage and range. So, the importance of maintaining your air pressure is more about load carrying capacity of your tires. If you wait to check your air pressure until your TPMS light illuminates on your dash, do the math!