Tires: Tire Pressure, mitsubishi montero sport, tire placard

Question
I have a 2002 MITSUBISHI MONTERO SPORT XLS, my son just put 4 new Firestone 10 ply tires on it (we live on gravel roads, but drive some highway). I believe he said maximum psi is 80 lbs, but that he put 45 in them to see how that felt and drives.  It drove the 12 miles home fine.  My question is if we are going on a 2 day trip with highway driving do I want the pressure increased to say 60 or 65 or more?  I am confused on which makes more of a roll over risk, lower or higher lbs of pressure.  Lower tire pressure may make a smoother ride - but I don't care about that, safety is my first concern and then possibly tire wear and gas mileage.  Maybe we should have only put on 6 ply? But the 10 ply were the same price or a better tire, I think. So my question is what should we set the tire pressure at?  And next time should I buy different tires?  Thanks!

Answer
Neva,

On every vehicle sold in the US – and I am told this is worldwide, but I have not been able to verify this - there is a sticker – commonly called the tire placard - that lists the original tire size and the proper pressure for that size.  The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.

Personally, I like a car that handles crisply. So I use 3 to 5 psi above the placard pressure.  I also get better fuel economy, better wet traction, better snow traction, better tire durability, better tire wear, but I do have a degradation in ride harshness (but I am willing to put up with it.)

BTW, it doesn't matter who makes the tire or what pressure is listed on the tire's sidewall, if the tire size is the same as the placard, then the pressure listed on placard is also appropriate.

One word of caution:  Some trucks use tires with the letters "LT" in front of the tire size - and some trucks use tires with the letter "P" in front of the tire size - and some trucks use tires with the letter "C" after the tire size.  You should not use "P"'s in place of "LT"'s and vice versa.

I looked up your vehicle in a book that summarizes vehicle tire placards, and it lists a P type tire for your vehicle.  But "10 ply" means it is an LT type tire, which is quite different than a P type tire - and more importantly, LT type tires do not come in the same size as your original tires, so I will need to know all 3 items from your vehicle:

1)  The original tire size listed on the placard (be sure to include the letters!)

2)  The inflation pressure listed on the placard

3)  The new tire size currently on your vehicle (be sure to include the letters!)

With that information I can answer your question.  Without it I am only guessing - never a good thing to do!