How To: Figure out LT tire pressure

[Matt]

This has long been one of my pet peeves across many internet forums where people discuss tire inflation pressures. Too many people really have no clue how to figure out their tire pressures, and it's not too difficult.

The #1 misconception is that you just put the "max" that it says on the sidewall. Horrible idea! My favorite example of this was a guy that argued with me that he's supposed to run 80 PSI his E-Rated tires that he put on his Geo Tracker. Somehow, in his head he thought that he needed to run the same pressure as a fully loaded 3500 series truck...

So how do you do it?

There's a few ways to go about this. You can look up the pressure VS load chart for your P rated tires. BUT you must de-rate them by dividing by 1.1!!
The OEM Commander tires, P245/65/17, have a load capacity of 2100 lb @ 35 psi.... and we need to divide 2100/1.1 = 1909. So 1909 LBS is the actual load you're looking for. Now you can go to the LT tires inflation vs pressure chart, find your size and find the correlating pressure for your desired load. Keep in mind, this is also the "fully loaded" pressure and you probably don't need to run this high 100% of the time... so there's another method i discuss below that I prefer.

Unfortunately, the TRA (North American Tire & Rim Association) doesn't have EVERY size tire and pressure listed in the chart im providing at the bottom... actually most Passenger tires in this chart only goes up to 32 PSI even though most trucks and SUV's recommend 35 PSI.

So, here's what I recommend doing:

Look at Curb Weight (CW) and Gross Axle Weight Rating (GAWR) to figure out what range of pressures are good for you.

Curb Weight:

Take the curb weight (CW) of your vehicle, plus about 1000 and divide by 4.. then round up. Using this "load", you'll have an idea of the ABSOLUTE MINIMUM pressure you should run in the tires. Taking my 07 Commander as an example, the CW is somewhere around 5200 LBS. So to be a bit safe, add 1000 which brings it to 6200 LBS and divide by 4, which gives me a load of 1550 LBS on each tire. I'd then round that up to the next hundred to make my minimum target 1600 lbs.

Now I go to the load vs inflation table for LT tires. You'll notice, the minimum pressure given for any LT tire is 35 PSI, so you just want to make sure that 35 PSI is a good "minimum" pressure to run. I run LT245/75/17. You'll see that these tires have a load of 1770 LBS at 35 PSI... so im definitely good running 35 PSI for daily driving.

GAWR:

The GAWR is the maximum load on the axle... anything above this and you're overloaded. In most cases, the combined Front + Rear GAWR is more than the GVWR. So while some people prefer to use the GVWR to find the MAX pressure just like I used the curb weight to find my required pressure. Note that you want to use the REAR GAWR since it is higher than the front rating.

Here's why I use GAWR instead of GVWR:
My GVWR is 6500.. divided by 4 this gives me a load of 1625 LBS per tire. Looking at the chart for my LT245/75/17s, the max pressure i would ever need to run via this method is 35 PSI because that will take a load of 1770 LBS.
BUT, If i take my REAR GAWR of 3650 and divide that by 2, I get 1825 LBS. Therefore, if my Commander is fully loaded and im only running 35 PSI, i could be overloading my rear tires. According to the TRA chart, I should run 40 PSI these tires; This correlates to a load of 1945 LBS which covers my rear GAWR.

Now if im fully loaded, i can make the decision to bump it up to 45 PSI if i want. But at least now, choosing a tire pressure to run is based on an it's an educated decision. as opposed to "eh, the max on the sidewall says 80 psi so i should run 60 psi right?"

I run 35-38 PSI for daily driving, and about 42-43 PSI when i'm loaded. I'll bump the rear up fo 45-50 if im towing a heavy boat.

Here is a link to my "go to" PDF: http://marktg.toyotires.com/file/loadinflationtable.pdf
P-Metric Load Vs. Inflation charts start on PDF Page 14
LT Load Vs. Inflation charts start on PDF Page 20
FYI: This is a Toyo document, but TRA pressures are the same regardless of brand. These pressures are the same for Cooper, Hankook, Bridgestone, Mickey Thompson, etc, etc.

If your tire size is not listed in this chart, send an E-mail to the manufacturers tech support department and they should be able to E-mail you a copy.

Hope this helps!!

[Sal-XK]

That's pretty cool and useful information thanks for taking the time to post this.

[Adondo]

Good info, and this comes up all the time on the RV.net forums.

Take my motorcoach for example. The rear axle weighs 17,800 lbs., the front 9,600 lbs. Just because the Toyo M120z tires which are mounted on 22.5'' wheels say 120 PSI max doesn't mean they need to be blown up to that amount. According to Toyo's charts for the tire's particular size, the rear tires (the PSI pressures per weight are different if they're dual) run 100 PSI. The front axle tires run 105 PSI. That's a helluva lot less than the 120 PSI max. If they were maxed, they would wear out in the tread center.

Commercial vehicles which can include heavy RV's, always need to be weighed and a tire manufacturer's chart consulted before just guessing as what to put in a tire. It shouldn't be any different for our Jeeps. Especially the Commander for that matter, as it's a heavy rig. My XK weighs a bit more then our Ford Expedition in fact, despite being a smaller vehicle. An XK is a stiff as a Tonka Toy for a reason. (Very little body twist)

[fosh69]

So question: has anyone weighed the front and rear to see what the weight over the axle actually is? I have a 5.7 over the front, and other than a few pounds here and there for add-on's, I would think it's going to be close to the same all the time, right? The load in the cargo area may affect front weight over the axle, but probably not by much. As a rule, being a truck guy, I have always run more pressure in the front tires (usually about 2psi) unless I'm loaded. Any input on this? Thanks.

[Matt]

I used to run 3-5 PSI less in the rear tires of my pickup truck as well.

For my Commander i run them all the same pressure. I'm not sure what the exact weight distribution is on these vehicles, but it looks to be pretty close to even.

My "standard" procedure is to run them all the same unless i see signs that pressure should be reduced, ie: the center tread of the rear tires wearing more, or non-uniform tread contact patch.