Epistle on Proper Tire Inflation

[hufhouse]

About thirty years ago, when my dad was teaching me about cars, I seem to remember that most tires said something like "Inflate to 30-35 psi." So, Dad always said to inflate them to 32 psi.

In recent years, I've been taking my car to my tire guy for oil changes, and I rarely...ok, never...check my tire pressure. He checks it every 3,000 miles.

I've been having issues lately with tire wear on both of my vehicles. I've fought tire cupping on my '02 Saturn L300, and I've had uneven wear issues with my '00 Pontiac Montana. On the Montana, I recently replaced the tires with SUV tires that my tire guy recommended for their long wearability and ride.

I've noticed that tires NOW say something like "Maximum tire pressure is 44 psi at maximum weight of xxxx lbs." I've done some internet research, and I've found that tire manufacturers recommend inflating tires to the exact pressure shown on the label on the door of the car. One website gave very strict instructions about inflating the tire while it is cold to that exact pressure.

My L300 has a recommended inflation of 30 psi. My Montana has a recommended inflation of 35 psi.

My guess is that each car has its own inflation recommendation based upon the weight of the vehicle and the "footprint" that the properly sized tire makes on the ground. Inflating the tire to the correct pressure puts even pressure on the entire footprint (not more on the edges or in the middle) and allows for proper handling and control.

Here's my question, though. The tires I just purchased for my Montana have a maximum pressure of 53 psi. I read about the tires online, and other people who own this tire say that it has to be inflated to a higher pressure to avoid wear on the outside of the tire.

Is it possible that a different type of tire...say one with a very stiff sidewall...would have a different pressure recommendation than that shown on the sticker on the door?

I just want to make sure that I'm doing the right thing. This is a variable that I want to eliminate when it comes to good ride and good tire life.

Does anyone have any expertise in this area?

[1999montana]

Originally Posted by hufhouse

...I've noticed that tires NOW say something like "Maximum tire pressure is 44 psi at maximum weight of xxxx lbs." .... My guess is each car has its own inflation recommendation based upon the weight of the vehicle and the "footprint" that the properly sized tire makes on the ground. Inflating the tire to the correct pressure puts even pressure on the entire footprint (not more on the edges or in the middle) and allows for proper handling and control....

I think your reasoning is pretty much on the money.

For what it is worth, my van has Michelin MX-4's on it with almost 100,000 Kms and about 35% tread left. They are rated to 44 lbs max, but I never go beyond 35 lbs. I check them once a month, or if they look 'low'. With the cooler weather coming along soon, I will check more often and add air to bring them back to 35 lbs.

This is somewhat a personal choice. At 35 lbs, the ride is firm but not harsh and you can feel most of the imperfections in the road surface. Fuel economy is best for me at 35 lbs. (less rolling resistance).

I base my experience on how evenly the tread wears across the face of the tire. If a tire is under-inflated, you will wear the shoulders more than the center. If it is over inflated, the center will wear prematurely, there may be cupping, but in addition you can also set the tire up for ply separation and possibly total failure if the pressure is too high.

At speed and under full load on the highway, the tire sidewall will flex with each revolution. The heat generated will be dissipated reasonably well if the tire is properly inflated. If under-inflated the heat build-up can be so severe that the tire experiences a sidewall blowout at speed.

After a long run it would not be surprising to have a tire accurately inflated (cold) to 35 lbs show a reading of 44 lbs hot.

I’m surprised that the SUV tires have not created other issues with traction control or ABS. I wanted to go to an SUV tire with a raised letter to replace the OEM tires (Generals) but three different tires shops and the tech at Michelin on-line would not recommend them for this reason. The ones I wanted wear about a 1/2 " taller and consequently had a different diameter.

My thought on proper inflation, particularly with a stiffer sidewall tire would be to start with GM’s recommendation and then go up or down a few pounds to see how the ride and handling changes. I don’t know if I would go beyond 35 to 40 lbs though even if the rating says 53 lbs.

Just my opinion.