Tires: max load at psi?, Tire Load Carrying Capacity

Question
What is meant by 1550 lbs max load at 35 psi? Most midsize vehicles weigh more than 1550 lbs. Or is that per axle weight? How do you figure?

Answer
Larry,

The figure you referred to is the load carrying capacity of the tire - and in this case, it is indexed to 35 psi.  

All tires sold in the US are required by regulation to indicate the maximum load carrying capacity and the maximum usable pressure - and most of the time those 2 items are not connected.  This is governed by a regulation that is a bit vague and there are different interpretations as to what is required.

However, this is also controlled by tire standardizing organizations.  While the tire standards are not exactly the same throughout the world, they are close enough that you can consider a particular "size" to carry the same load at the same pressure.

This is convenient (and purposely done this way) because on every vehicle sold in the US, there is a sticker - commonly called the placard - which lists the original tire size and the proper pressure for that size.  This is also a government regulation and is determined by the vehicle manufacturer.

The derivation for the tire standard (and therefore that load that is imprinted on the sidewall) is a table - commonly referred to as the "load table".  It is a somewhat ideal figure - smooth surfaces, moderate speeds, moderate temperatures, etc. - and recent events have pointed out the shortcoming of the load tables.

The vehicle manufacturers and the tire manufacturers both realized that they could not change the way this is done without completely scrapping the tire size methodology, so the vehicle manufacturers were encouraged to oversize the tire (which most of them were doing anyway, and recent events encouraged then to go further).

So back to your question:  The 1550 # is for the tire - and since there are likely 4 tires on your vehicle - the absolute maximum the vehicle can weigh is 6200# - that is, fully loaded and no aerodynamic devices to increase the load at speed, plus the operating conditions have to be pretty close to "ideal".

Vehicle manufacturers use a tire larger than the minimum - and that just is good engineering practice.  However, the methodology to determine what they use is considered a trade secret.  We tire engineers have developed a term called "Reserve Capacity" to help us get a grasp of what a particular vehicle manufacturer is doing.

So it is quite likely that your vehicle is rated to only be about 5,000 # when fully loaded - GVWR (Gross Vehicle Weight Rating)

The short answer is that you should use the vehicle placard for pressure if you are using the same tire size as listed on the placard.  If you are resizing the tires, use a tire that has the same load carrying capacity as the original.  The "Load Index" is an easy way to determine if the tires are the same capacity.