Tires: Tire Press, tire placard, inflation pressure

Question
QUESTION: We bought a used 1999 Suburban K1500 4WD with P265-75R16 tires.
I don't know the proper pressure for normal driving with lite loads.

Thanks

ANSWER: Tom,

On every vehicle sold in the US, 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.

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.

If you can not find the vehicle placard, post back and I'll try to narrow it down as your vehicle description doesn't quite cover all the possibilities, so the vehicle tire placard is the best location to find the correct information.

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

QUESTION: THANKS for your response!
The sticker shows LT245/75R16C 45psi cold, but it has P265/75R16 tires now.
I hope this allows you to do the calculation. It appears, from your comments about changing tire numbers, that the previous owner did not receive good information when he replaced the tires.
Thanks again, Tom

Answer
Tom,

Thanks for the additional information.

At 45 psi, an LT245/75R16 has the load carrying capacity of 2280#.

In order to carry the same load, a P265/75R16 must use 33 psi.

But there is one additional problem.  The stock wheels on 1999 Suburbans was 6.5" wide, and a P265/75R16 requires a minimum of a 7" wide rim.

Using a rim that is too narrow causes the tread to arch across the tread face and may, in turn, cause the vehicle to try to follow the grooves and ruts in the road surface and make the vehicle sensitive to the wind.