hello all. i have a '99 F-350 V10 4X4. great truck. i have a question about what the correct tire pressure should be. the door jam says that for 265/75/16 it should be 45 front, 75 rear. i bought the truck used, though, and it has 285/75/16 tires on it, and the tire sidewall says max 65psi. i use the truck to tow my travel trailer (about 5000 lbs loaded). so, i was wondering what you guys thought the ideal tire pressure should be both when i am towing and when not towing. so far, i have set all four tires at 55psi for all occasions and it seems to be doing fine, but i thought i'd ask since i'm not sure if i am doing the right thing. (BTW, the tires are General All-Terrain LT. i know people are going to say those tires are crap, but since they were new when i got the truck, and they have been doing fine, i don't want to shell out the $$ for new tires since it would be pretty expensive.)

 

Just some info for ya.. I'm paying between 600 and 650 for 4 285/75R16 D's on my 02 F350 4x4 PSD. As for the air pressure (I known mine's a diesel...) but I run 50-55 in the fronts and 45-50 in the rear unloaded. Then depending on my load, I'll run 60 in the fronts and between 55 and 75 in the rears. I too run load range D tires instead of the stock load range E and my sidewall also says 65 PSI Max, but running the higher pressures was by the recommendation of my tire guy. They wear great and handle awesome!

 

Just my opinion but if your running load range D and your hauling with them then I personally would have the atleast the rear tires at the max air pressure that they are rated for and probably all 4 maxed.

It is safer for towing and it will basically strengthen/increase the tires sidewall and load handling ability plus ,it won't wear your tread down as much. When your not towing 52-55 pounds would be the o.k. because there is less of a load bearing on them.

I personally run my tires at the max air pressure rating that the tire is rated for towing and not not towing. The ride is a bit harder and more uncomfortable but the tires last longer period,they wear more evenly.
Now winter driving air pressures are a different story.I would run less because you want a softer more agressive and more forgiving giving tread that can give and take and not be so abrupt to ongoings/instability from hitting a bump on slushy or snow laden roads.Winter (cold )causes tire rubber to be harder than summer so, run a little less air for that reason.

Summer( hot) causes tire rubber to be softer and wear quicker so add more air or keep at max if you like your tires to last longer and don't mind the harder ride comfort.

 

Use the chalk line trick. Draw a line across the tread, roll maybe 15 or 20 feet, then read the line to see how much of the tire contacts the road. High tire pressure will give you better gas mileage, but might also wear down the center tread without a load.

 

I have not used the chalk method yet.I've had to learn the hard way through feel trial and error I get great even wear by my air judgement..

The chalk method does seem like the best logical way to judge for even tread wear..

 

In general, yes. The chalk-line method will definetly give you the most accurate wear pattern for the given load / temperature combination. One you add any load, your wear pattern will change. And as jaybird mentioned, temperature also plays a big role. Not only ambient temperature, but tire temp as well. As you travel, your tire will heat up due to friction, increasing both tire temp and internal air temp.

I suppose the best method might be to do the chalk line test, with the typical EVERYDAY load (most likely unloaded), on an average temp day, after driving down the freeway for atleast 30 mintues. Then note the proper PSI's for each tire and write them on a label inside the gas door next to the factory label. That way when you adjust for loads or whatever, you can readjust to optimum when you're done.

 

Here’s the proper way to determine what pressure to run in your tires. Weigh your truck, getting individual weights for each wheel (or at a minimum each axle). Then consult the tire inflation chart for your brand/model of tire. You can view a chart at www.trucktires.com . The chart will tell you what pressure to run based on your weight. If you got individual wheel weights, use the heaviest wheel on an axle to determine what pressure to run on all tires on that axle. If you weigh your truck unloaded you’ll need to do this again with your normal load. If your load fluctuates all the time, it’s probably easier to just keep the tires inflated to handle the highest load you anticipate.

In my case, I carry a 4000 lb slide-in camper occasionally. I keep the truck tires inflated for unloaded driving most of the time. Then before I load the camper, I pump them up for the extra weight. That way the truck rides smoothly with good traction and even tread wear while unloaded. If I was loading and unloading the camper daily or even weekly, I’d probably just keep the tires pumped up. Changing tire pressure daily on a dually would get old real quick.
It's a good idea to read the footnotes and fine print in tire inflation manuals also. For instance, in the Firestone manual it says that if you are driving for long periods at 65 to 74 mph while at the rated max load for the tire, you should increase the tire pressure 10 psi (provided the wheels are rated to handle the pressure). In the case of the Firestone Steeltex 265/75-16 tire that would be 90 psi. This means that the 80 psi listed on the sidewall is not the max pressure. If you want to drive 75 to 84 mph , you still need those extra 10 psi, but you also need to reduce you max weight capacity by 10%.

You can view a tire inflation table for LT235/85-16 tires at www.trucktires.com .
Here’s the inflation table for the LT265/75R16 Firestone Steeltex tire

Weight Pressure
1910 35
2100 40
2280 45
2470 50
2625 55
2790 60
3000 65
3105 70
3260 75
3415 80

65-74mph add 10psi
75-84mph add 10psi & decrease load 10%

 

Harold - Okay, I see you have a DRW. According to this chart, if your rear axle weighs 2000 pounds, what pressure per tire should you use?

 

That's a good question. Although my rear axle weighs more than 2000 lbs unloaded, the weight per tire is still below the chart. So I just run 35 psi in each rear tire when I'm not hauling. This is higher than "necesary", but I can't find any specs that allow lower pressure for normal driving. None of the tire manuals I've read address this issue.

 

So the question becomes, if you draw a chalk line on it and discover that only the center three treads are making ground contact, what to do? To answer this I would think you need to consider the degree of sidewall flex the manufacturer is comfortable with in dually use. So take the max rated psi at the max rated load, and divide the psi into the load, which will leave you with the square inches in contact with the ground that the manufacturer expects at full load. So unloaded, any psi that results in that amount of ground contact or less, should result in an acceptable amount of tire flex, particularly if it contributes to even tread wear.