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Oh, one thing I forgot to mention that I wanted to talk about.

So, I still have P rated tires. Since I have a marginally rated tire, conventional wisdom says air that puppy up to max, right?

Well, that's what I've always done. However, a while back I stumbled upon a little internet argument on the subject. One fella was vehemently defending NOT doing that. Claiming he had been in the tire industry for many years. Designing and producing.

He said if you pay attention to the inflation tables, P rated tires carry their MAX load at something like 35 or 36 psi (which, SURPRISE, is usually about what's stated on our door sticker to run our tires at). As the chart goes on, the tires pick up no extra capacity at more psi than that. He said that all you will accomplish is ballooning out the tire a bit, wearing the center of the tire more and reducing your contact patch. Even while towing. And effectively make them tow WORSE.

I looked the chart up, and sure enough, the tires carry their max load rating at 36 psi or something like that, even though they have a max psi rating of 44. I've always followed what I thought was the conventional wisdom of airing my tires up to max for towing duties if I was towing heavy and getting close to max specs.

Most of the folks in this discussion wrote the guy off as a kook, but after looking up the inflation tables for myself, seeing that they did indeed carry max weight at only 36 psi, and considering that Ford does indeed have that as the recommended pressure on my door sticker, I thought I'd try it.

I decided to run the front tires at exactly that spec on the door, since they are carrying exactly the same thing they do unloaded. I ran them at 36. I still couldn't completely shake the old habit of airing up more in the rear while towing, so I aired the rear tires to 38. I thought you know what, I'll try it. If I don't like it I can pull over and air them up. I keep my Ryobi 18v compressor under the back seat at all times.

I swear to you that the truck felt a bit more stable and less squirmy this trip. Could it be placebo? I guess it could. But it DEFINITELY didn't feel worse. Last couple of times I went out I had them aired up to max. I won't be doing that again. I'll just run at 36 and 38, and I'm sure I'll be sticking with P rated tires in the future. Unless the size I'm looking at isn't offered in it (I may step up to a 33 next go-round).

So, call me crazy, or believe it, or be skeptical but try it for yourself. That's your decision. But, I SWEAR it was a better feel in the truck. I was a little doubtful myself, but I liked it.

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I usually run my tires the same pressure as unloaded, but I did start running 40 psi all the time when I got my 275/60R20 Duratracs because they were a little more floppy even unloaded. The extra pressure did get them to feel more like the OEM tire. I wanna say my Duratracs are rated for 51 psi max though.

The same argument is not true for ST tires that you would have on your trailer BTW. Those definitely increase in capacity right up to the maximum pressure.

There is also some weird guidelines around inflating your ST tires if you use them above 65mph. ST tires were originally designed and specificed for 65mph operation but the newer tires are rated as high as 85 mph. This is what Maxxis says:

From 66 to 75 MPH - the tire inflation pressure needs to increase 10 PSI (not to exceed the maximum PSI the tire is rated for) and load should be reduced by 10%

From 76 to 85 MPH - the tire inflation pressure needs to increase 10 PSI (not to exceed the maximum PSI tire is rated for) and load should be reduced by 10%
So the way I read that is that your trailer tires need to have 20% loading overhead in them to handle the weight of your trailer at >75mph. And you need to be able to put an extra 20psi in the tires without over inflating.
 

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Discussion Starter #42
My trailer tires are "D" rated and have a 65 psi max pressure. I have been running them at 65 psi, and they are speed rated to 87 mph. I'll look their ratings back up and crunch the numbers based on what you've just posted.

The camper had C rated tires to begin with. Those just barely added up to the gvwr of 7500 lbs. I know you subtract your tongue weight out of that to get what the tires are carrying, but that was too close for comfort to me. I replaced them with D range tires.

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Those are interesting notes. I guess I’m right on track having my trailer tires at 75lbs for an 80lb max tire, and my truck at 36lbs on an 80psi tire. Sounds wrong but has been working for me for two years now on my 18.


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Discussion Starter #44 (Edited)
My trailer tires are rated at 2149 lbs each, and 81 mph (not 87). I run them at 65 psi, and I run 65-70 mph. I will run it up past 70 of necessary for passing or something, but not for sustained periods. Usually have the cruise set on the speed limit, up to 70.

I'm carrying about 3000 lbs a piece on the trailer axles.


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My trailer tires are rated at 2149 lbs each, and 81 mph (not 87). I run them at 65 psi, and I run 65-70 mph. I will run it up past 70 of necessary for passing or something, but not for sustained periods. Usually have the cruise set on the speed limit, up to 70.

I'm carrying about 3000 lbs a piece on the trailer axles.


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Yeah, I just went to D range but my trailer is only a 5500 lb gvwr.

I also run 70 mph max sustained but will go faster if i am passing and dont wanna hold up traffic. My trailer had Goodyear Marathons and suspect i was pretty close to maxing those out speed wise.
 
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