Why are they de-rated?

I put tires rated to 1500# on a car  and they are good to 1500#, if I
put them on a pick-up that weighs the same as the car they are de-rated 10%.
Why is 1500#  not  1500# anymore?

I don't see why a tire inflated to the max printed on the tire would
be worth less (pounds of vehicle weight) if the vehicle it's on has a
different shape.

I do recall that LT tires can be inflated 10% over the printing
because they are LT tires

Does anyone know why?
Thanks.

Rob
becida@comcast.net

At  10/16/2011 04:08 PM, Dennis Haynes wrote:
>Hopefully those 215/65's are reinforced or XL rated. Otherwise they are at
>the lower limit of acceptable capacity for the Vanagon. Especially if you
>subscribe to the 10% de-rate when using passenger car tires on light trucks.
>
>
>Dennis