Tires: Tire Pressure for Suburban with Light Truck Tires, tire placard, light truck tires

Question
QUESTION: I have Goodrich All Terrrain T/A tires (31x10.5R15LT) on my 1988 GMC Suburban 1500. It has a heavy duty suspension and rides really jarringly. Use is almost all freeway, some mountain roads, and not heavy loads (no towing). What is the lowest tire pressure I can run to soften the ride?

Thanks,

Dennis
PS Can the answer please be emailed to me also?

ANSWER: Dennis,

On every vehicle sold in the US - and I understand this is a worldwide practice as well - there is a sticker – commonly called the vehicle 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.

For this situation, you should be sure the pressure on the sidewall equals or exceeds that pressure on the placard.  If you do this the issue about the letter "C" in the sidewall will take care of itself (which is not true with "LT"'s vs "P"'s.)

If you are using a tire size that is different than what is listed on the placard, then the pressure has to be recalculated.  The calculation is not difficult, but it requires tire load tables which are not allowed to be published in the Internet because of copyright laws.  However, I have a copy and would be glad to do the calculation, but the starting point is the vehicle placard: Tire size and inflation pressure.  I will also need to know the new tire size.




---------- FOLLOW-UP ----------

QUESTION: Thanks for the reply. Unfortunately the placard is unreadable.

Also, it would not give pressures for the current tires as the Suburban came with car tires (235- or 255- something (I don't recall)) and I have switched to the light truck tire, completely different.

So, for the current tires, I need to know appropriate tire pressures and what the lowest one that can be used. With the HD suspension, the ride is horrible at higher pressures.

If you could please calculate for me what the tire pressure range should be, I would greatly appreciate it (lowest to highest for the LT tire).

Thanks again,
Dennis

Answer
Dennis,

The starting point for calculating the pressure of a different size is what is listed on the vehicle tire placard.  I have a book that summarizes vehicle tire placards, but not only is it common for vehicle manufacturer to supply different sizes of tires, the book is sometimes wrong.  I always like to know what the placard says.

According to Tire Guides, a 1988 Chevy Suburban came with a variety of tire sizes (as I mentioned above), but taking an educated guess based on what you have told me, the vehicle tire placard probably said the original tires were P235/75R15 XL inflated to 35 psi front / 41 psi rear.

In order to carry the same load, a 31X10.50R15LT needs to use 38 psi front / 42 psi rear