In this series I am giving a lot of detailed technical stuff. For some of you this will make your eyes glaze over with boredom while others really want to know the “Why” of different tire selection. Please bear with me as I try and build a foundation of information that anyone can come back to later if they develop an interest or have a question.
Last time we discussed “P-Metric” sizes. That means tires whose size begins with the letter “P” and have metric width dimensions. P235/75R15 the 235 is millimeters and the size starts with a P so that is a “P-metric” size.
In the P-metric post we discussed a P235/75R16 105S which was rated at 2,028 Lbs at 35 psi max. Before we move to Light Truck tires where things get a bit more complex with multiple Load Ranges we need to finish up our discussion of Passenger tires. These are “Standard Load” tires with a max inflation of 35psi and “Extra Load” with a max inflation of 41 psi.
Since most passenger tires are standard load, that is the default so few tires actually have the words “Standard Load” on the sidewall. However if the tire is rated for Extra Load it will have the words “Extra Load” and possibly “XL” on the sidewall in addition to the 41 psi max information. Originally Extra Load passenger tires were used on Station Wagons that some of us might even remember. In the 60’s there were regular passenger cars and Station Wagons.
SUV’s, Crossover or other type vehicles really were not part of the market.
Since these larger vehicles were intended to be used with higher loads most of the time, it was soon discovered that there needed to be an adjustment when a tire was selected. Industry guidelines indicate that the vehicle normal load on the tire shall not be greater than 88% of the max load capability of the tire.
Additionally if a P-metric tire is used on a “multi-purpose” passenger vehicle, think SUV etc or truck, bus or trailer, the load rating shall be reduced by dividing by 1.10. The total load capability at the recommended inflation shall not be less than the Gross Axle Weight Rating or GAWR.
I guess you can see that things get a bit involved for the engineers at car manufacturers when they try and select an appropriate tire size and inflation.
Extra Load rated tires are one option used by the engineers when selecting tire size. In our example size P235/75R15 108S XL we can carry an additional 308 Lbs on each axle at max inflation.
If your tire placard or owner’s manual indicate your vehicle should use P-metric XL tires then that is what you should use.
I’ve got a short list of reader questions to answer, then we can move on to other type tires that many will find on their Pick-Up, Trailer or RV.