---- Rob <becida@COMCAST.NET> wrote: > 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?

This is the first I have ever heard that LT tires can be safely inflated above their rated maximum pressure. When the tire manufacturer says "maximum inflation pressure," I assume that means the same as "maximum inflation pressure."

The reason that the maximum safe load that car tires can carry is 10% less when installed on a vehicle calling for truck tires is because trucks are used differently from cars. Trucks are more likely to be driven on rough roads or unpaved areas. They are likely to carry the maximum load a larger percent of the time. Their use is simply more demanding. With the VW Campmobile, the kitchen and poptop installation itself brings the van up to near maximum capacity. With passengers and gear, there you are. With a car, the maximum load is not normally reached (how often do you carry a passenger in each seat with a loaded trunk?) Every time the vehicle is loaded to maximum, it puts maximum programmed load on the tires. Do this over and over, and any damage acccumulates.

The truck tire structure, with extra strength in the sidewalls, is different from the passenger car tire structure. The maximum load does not depress the side wall as much when the truck rated tire is inflated to carry the load on a truck as a it would on a passenger tire with the same load rating. To see this, equip two identical vehicles with truck tires and passenger car tires with the same load rating. Inflate both sets of tires to their maximum pressure. Load them up with the same load. Look at the tires. Notice the bulging sidewall on the passenger car tires compared to the more normally contoured truck tires.

That 65 psi, vs 44 psi or so (typical maximum pressures for truck and passenger car tires respectively), and the extra strength in the sidewalls make a difference.

mcneely