I'm going to be going from the crappy P265/70/17 tires on my '03 S-Crew to a LT tire. I haven't chosen the tire yet, but I'm likely to change to a LT tire because I pull a 7500# trailer with it and the P tires just aren't up to the job. How does one determine the proper tire pressure for the new tires? Certainly the door jam sticker which says 35psi isn't going to apply any more with better 'truck tires' on the vehicle. Man do those tires on the truck now suck for hauling. Wow.

 

Actually, it's the load and width of the tires that mainly determines the PSI needed, and when not loaded, the LT tire of the same width won't necessary need higher pressure then a P tire. (assuming, the P was inflated properly in the first place). Having said that, I usually run 40/40 (vs 35/41 on the door), with C rated tires, mostly for fuel economy. And I think the 2nd number id for a full load, which I hardly ever have.

There was a procedure with chalk posted here a couple of weeks ago -- it's kinda involved and takes a while to do it, but it can give you good result.

Personally, I just check the temperature of the tires when carrying a load, and inflate them accordingly -- 40/50 with lots of paving stones in there once, and I don't think you need more than that, either.

 

Where those P's on it from the factory? You will probably be amazed at the difference with the LT's. I would go with the LT's since you are pulling a trailer. Ask your tire dealers about the pressures to run.

 

Ya... P's from the factory. Goodyear Wranger RT-S... but they are a P instead of a LT tire. Max pressure on the tire is 35. Useless for anything except 4 people and luggage. Not bad in the Canadian winters at least... but the moment the truck is loaded the tires squat really bad and they really don't have the right stability for towing at all.

 

When you go to LT tires make sure the rims will take the extra PSI you may need for LT's. You state you are towing a 7500 lb TT. Most factory rims are only rated for the tires that are listed on the door jam and LT tires have a minimum and maximum inflation for their rating.... You stated the current max PSI was 35 for the big P rated tires. Most C, D or E rated LT's will require 50-60+ PSI inflation. I had a E-150 and wanted to put LT tires on the rims but they were only rated for a max PSI of 41. I went up in size and put on extra load rated "P" tires. The E-350 van I have now has E rated tires and calls for minimum inflation of 50 front and a max of 80 rear if I am at maximun load rating for the vehicle. Be cautious of what a tire shop will sell you. You may want to look at these sites before you make a decision - .tirerack.com and .1010tires.com ..

 

Thanks, I'm definitely trying to research this a bit. So I have already looked at a lot of tires at the tirerack site and looking at all their specifications on the sites. Then the next question is, how does one find out what loads the OEM '03 17" wheels are capable of carrying?

 

Good question, but not easy to answer. I have seen the max PSI stamped inside the wheels of some after market ones, not really sure about stock wheels. You could try your local Ford dealer, if the wheels are a standrad Ford part the Parts people may be able to give you this info. Tire Rack also has some numbers to call for info, thats another option.
You may also want to check the side walls of the tires on your truck and see what the load is for the P rated tires you have. With some LT tires the capacity could be less or more depending on the load range, the only difference being the stiffness of the side walls and the PSI required. You don't want a replacement tire with less carring capacity than you already have, since you use it for a tow vehicle..

 

I've compared my current tires, load rating 113 to all the available tires from Tire Rack, and they are all similar 113 load tires. All the 113 tires are rated to 2535lbs at 35PSI. These are all P rated tires. There are a few Load E tires out there.

The 121/118 Pirelli Scorpion, Micheling LTX/AS, and there is at least one bridgestone and a toyo that are rated at 3195lbs at 80psi (all Load E tires) - there are a rather limited number of Load E tires in my size. Looks like that's what I'll probably go to - but that will make for a harsher ride, and probably slightly worse winter grip. So I'll just have to drive slower in the winter.... big deal. But worth the peace of mind when towing. Then I just need to figure out the tire pressures on the tire. Sigh.

 

I went from Goodyear HT LR-E 245/75R/16 to the Mich LTX-MS LR-E for the E-350, They give a good ride, considering. In your case going from the P tires to LT E you will notice the difference. When not towing you can lower the PSI to get a little better ride. I got mine at Wal-Mart, I checked at Sam's Club and 4 other tire dealers, W/M was within $5 of Sam's Club, the others were $30-40 more per tire. The Mich are not a stock item but if you can find someone at the W/M's TLE that can special order them for you it should only take a week to get them in. One more thought, since the P tires you have are 2535lb and the E's are 3195lb, do you really need that stiff of a tire. Have you checked the load of the C or D tires that will fit, you may be able to go with them and still have load range to spare. They will not require as high a PSI, and should give a little better ride overall even when towing with their max PSI.......