Tires: Load Ratings, inflation pressure, tire vehicles

Question
Barry,

I have read through some of your answers to questions on this matter.  I have a 1/2 ton Silverado that says factory equipment was P265/75/16 S 35psi cold.  I was looking at going to an 265/75/16 load range C or LT since I pull a snowmobile trailer quite a bit.  Going through your responses to other people's questions on this subject you say to stay with a P metric tire if that was what the factory equipment was and that going to an LT or load range C rated tire has drawbacks or inherent dangers to it.  However you don't really go into any detail as to what the drawbacks are or why you favor staying with a p metric if that's what was standard vs going to a LR C.  I would think that running a LR C at the specified inflation pressure (50 psi) would be fine and give you a better tire if trailering frequently.

Could you elaborate a little bit?

Thank you.

Answer
Nathan,

You've more or less hit the nail on the head.  It's the 35 psi vs 50 psi thing.

The spring rate of a tire is pretty close to linearly proportional to the inflation pressure, so a tire at 50 psi (regardless of it's construction) is 40% stiffer than a tire at 35 psi.  Not only is the suspension designed for a tire with the lower spring rate - and that means it will properly damp out input for a 35 psi tire - the ride qualities of a 50 psi tires are much worse.  So the tendency would be to decrease the inflation pressure to get the ride back.

So what's wrong with running at 50 psi tire at 35 psi?  The same problem as running a 35 psi tire at 25 psi (same proportion!).  The tire was designed around a load inflation curve and using less pressure results in a higher risk of failure.

Besides, I don't see the point in going from a P metric to an LT metric tire.  Vehicles have a certain amount of load carrying capacity and towing capacity and the tire size is specified with these in mind.  So long as the payload and the trailer weight are within the stated capacities, then the tires will be OK.  However, if these are exceeded, then there are more problems then just the tires - the vehicle would need to have more capacity.