Tires: Tires pressure, tire placard, worldwide practice

Question
Hi
I have jeep grand chorkee, front tires lt225 75 16
Back ones, 225 75 16 what the tire pressure should be, some people say 40 some say 28-35 which ones

Answer
Umar,

You should not have different tires on the front vs the back, especially on a 4X4 or AWD.  

And while it may seem that the difference between LT's and metric's is trivial, it is not.  As a general rule, LT tires take about 15 psi more to carry the same load as a P metric - needles to say, this is going to result in a  different answer front to rear - and that is not good.

But let's start at the basics:

On every vehicle sold in the US - and I understand this is a worldwide practice as well - 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.

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.