Tires: Tire pressure/load correlation, bf goodrich all terrain, ford rims

Question
QUESTION: I have a 1996 Ford F-150 regular cab 8 foot bed 4x4 which currently has LT235-75-R15 Load Range C tires.  These are rated to carry 1985lbs at 50 PSI.  The recommended inflation pressure for these tires is 44 PSI on the rear axle and 35 PSI on the front axle.  I frequently haul heavy loads in the bed and tow trailers up to 6,500 lbs, so I run the tires on the rear axle at 50 PSI when loaded if I will be cruising at sustained high speeds for any length of time (I am no speed demon, I am talking about 55-70 MPH, depending on the speed limit).  Due to living in a rural area, and assisting on the family farm, I am looking to upgrade to a set of 31x10.5-R15 load range C BF Goodrich All Terrain TA Tires for better off pavement performance.  These tires have a maximum capacity of 2,270 lbs at 50 PSI.  They are also going on the factory Ford rims, which are 15x7" aluminum wheels, which is the minimum width recommended by BFG.  I want to keep tire wear even across the tread, so I know that I'll have to reduce the tire pressure accordingly.  I do not want to run under-inflated for the loads that these tires will see, so I need to know what front and rear pressures you would recommend for this setup, as I will be buying new tires in another month or so.  If the decision comes down to longevity or safety, err toward safety.  If you are able to estimate it, an unloaded and a loaded pressure recommendation would be even better.  I know that this is lengthy, but I also know that having as much information as possible will allow you to make the best recommendation.  Thank you for your time and consideration.

ANSWER: David,

Let's start at the beginning:

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.

According to Tire Guides, your vehicle should say the original tire size was ......  OK, there are several possibilities, but let me make an educated guess that the tire placard says the original tires were P235/75R15 XL inflated to 35 psi front and 41 psi rear.  Please check this as what I am about write is dependent on this being correct.

Notice the "P" in front of the tire size.  That indicates a certain type of tire designed to certain load / inflation characteristics.  One peculiar characteristic is that "P" type tires have to be derated 10% when used on a pickup, van, or a trailer.

Your LT235/75R15's were designed to a different set of load / inflation characteristics -and are more like the 31X10.50R15LT's you are considering.  These tires do not have to be derated as they were designed from the git-go for this application.

So in order to carry the same load as a P235/75R15, an LT235/75R15 has to be inflated to 40 psi front / 50 psi rear.

A 31x10.50R15LT needs to use 37 psi front and 41 psi rear.

I don't recommend differences in inflation pressure between loaded and unloaded conditions.  I think it is way to likely that there will be a situation where it is inconvenient to air the tires up - and since a tire never forgets it was operated underinflated, it is safer to always use the specified loaded pressure.

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

QUESTION: Thanks for such a quick response.  You have given me peace of mind with that answer.  I have had the truck for so long and have used LT tires I forgot that it did indeed come originally with XL passenger tires. For future reference, is there a tire pressure to load equation that I could use?  I would like to be able to fool around with the math, because I find that when I have an equation and can do the math, I understand it better.  Thanks again for your help!

Answer
David,

There is indeed an equation, but it is quite complex, so it is published in the form of a sereies of tables.  Unfortunately, the origanizations that publish these tables have limited resouirces, so they publish the tables in a book and forbid publishing the tables on the internet.  The book includes many type of tires and is quite thick.  It's also expensive - $60.00!

But I'll always have a book, so you can ask me to do another one - or if you want to know more about how this is done you can ask me questions - but before you ask the first question start here:

http://www.geocities.com/barrystiretech/index.html