QuestionQUESTION: I have a 2004 ford f150 lariat supercrew with bf goodrich all terrain tires lt285/65r18. What should the tire pressure be? The sticker on the door indicates 35psi for the stock tires. From my understanding the all terrains are load range e and may require a different psi. Note: the ford dealer that did the tire installtion set the all terrains to 35 psi.
ANSWER: Jason,
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.
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.
And that appears to be the case here.
Ford produces so many versions of their pickups that it is hard to know what the original tire size was unless you look at the placard. That's the only reliable source of this information.
So if you'll post a followup with what the tire placard says - size and pressure - I'll calculate what the pressure should be for the new tire size.
---------- FOLLOW-UP ----------
QUESTION: Thx Barry,
The tire placard on gas door says p275/65r18 and 35psi front and rear. As indicated in my original question I upgraded the tire size slightly to lt285/65r18 load range E. The psi I'm looking for is the one that will give me the most comfortable ride but still allow my tires to wear evenly. Also I don't haul any heavy loads so the tires don't need to inflated to the max psi as indicated on the sidewall. And I drive in all weather conditions as I live in Vancouver Canada. Thanks for your help.
ANSWER: Jason,
In order to carry the same load as a P275/65R18 does at 35 psi, an LT285/65R18 needs to use 43 psi.
Now you may ask why would a tire that is dimensionally bigger take more inflation pressure.
It has to do with the way in which the tires are designed. P metric tires are designed to be flexible, where LT tires are designed to carry more load for the amount of space they occupy. We could call that high unit loading.
When subjected to stress, rubber tends to be permanently deformed. In order to keep this under control for tires with high unit loading, the amount of unit loading has to be reduced. In other words, the rubber is not as flexible in order to carry the load.
So an LT metric tire requires more inflation to carry the same load.
And this is the basis behind how the tire is designed and all the things that impact wear are proportionally adjusted.
---------- FOLLOW-UP ----------
QUESTION: Thx for clearing that up and it makes sense why lt's require more psi. How much would the ride quality be affected by increasing the psi from 35 as set by my ford dealer to 43 as per your recommendation. And why would ford when installing the lt's inflate them to only 35 psi? Guess they don't have the expertise? And finally, are there any driving situationswhen I should consider inflating or deflating the tires? I just want to make sure that they give me thebestride quality but at the same time wear evenly.
AnswerJason,
There is so much involved with trying to predict what effect an inflation pressure change would have, that even car manufacturers will do ride comparisions - and not guess. So I'm not even going to attempt to make that prediction.
Why would Ford inflate the tires to 35 psi? I assume you meant the Ford Dealer - and if that is the case, it's because they just don't know any better - which is why I answer these types of questions.
I am a fan of a single use pressure - with the possible exception of soft surfaces - like sand!
