Biggles

Going to pick up my first TT in a few weeks. I have P rated SRA tires , normally set at 35 PSI . I am thinking I should add more air to the tires when I tow the trailer. TT will weigh about 4800LBS when I pick it up. I will be driving about 200 miles. The TT will have a WDH .
Would 45 PSI in all 4 tires sound about right ??

kozal01

iTrader: (2)

I run the max of what the tire is rated at when I tow which is 50 PSI on my LT tires. Look on your sidewall and see what the max rating is.

mbopp

I wouldn't go 45PSI in P tires. But I do fill mine to 35PSI.
When it's time for tires I may look for some ER (extended range P) tires. I think they're good for 44PSI.

BassAckwards

Actually P-Metric tires do not gain any extra load carrying ability beyond 35psi unless they are designated as XL.

If you see a P-Metric tire with a max pressure higher than 35, that pressure is only to achieve the speed rating printed on the sidewall.

So you can tow at 35 psi if you would like no problem. If bumping it up makes you feel more comfortable it won't mess anything up.

tomb1269

This may be technically correct, however from experience I always increase the tire pressure to the tire manufactures specified pressure marked on the tire when pushing the tire to it limits (i.e. towing). The tires ratings were set by the tire manufacture and I have to believe that to carry the tires max weight you need the tire fully inflated. I do know that 44psi tires that are kept at 42 to 44psi tend to last longer on the tread and wear the tread more evenly from side to side of the tire. Again, form experience a 44psi tire will preform better for handling at 44psi as well. The 35psi marked on the door is to maintain ride comfort not for the after market tires max performance.

BassAckwards

You are correct on all points except "that to carry the tires max weight you need the tire fully inflated."

You are correct that a P-metric will generally wear better when run at the max PSI because it will ensure an unfluctuating contact patch as long as the tire is on a rim of the appropriate width.

And yes the tire will handle bettter at max psi becuase there is significantly less sidewall flex so you won't get any "tire roll" when taking corners hard. The max PSI also must be followed to achieve the speed rating.

However 35psi is all you need to tow, now that doesn't mean you can't bump up the pressure, and that there may not be advantages to doing that. I just didn't want the OP to think "I have to Max the Tires" because you don't. If he has stock 18s with P275/65R18 tires they will hold 2600lbs a piece at 35psi. 2600lbs x 4 = 10,400lbs which far exceeds his ~7200lbs GVWR (His may be different, I'm just using that as an example as that is my GVWR, and any HD F150 would come with LT's so their GVWR is irrelevent to the discussion).

Here is a P-Metric Load Index Chart that will show you what I'm talking about.

Check at the bottom for the 18" size and the 65 series tires. Notice at the 35psi mark it says read Note A.

Note A states: "Some P-Metric Light Load and Standard Load Tires may be branded with a maximum inflation pressure of 35, 44, or 51 for high speed and handling optimization. However, there is no load increase above 35psi."

kozal01

iTrader: (2)

One thing to keep in mind though is heat. Yes a tire may be able to hold a rated load at a lower PSI but doing so under heavy load will induce much more heat into the tire on the road which can lead to premature failure. Heat is a killer of tires.

BassAckwards

Exactly! This is one of the advantages I was talking about in reference to bumping up the air pressure a little bit for towing. I too do it when towing just for piece of mind.

Again, however, it is not necessary as these tires are plenty strong for the load capabilities of our trucks. Lets just consider the rear axle for a moment. My 9.75 Axle had a load capacity of 3850lbs far short of the 5200lbs capacity of the rear tires. If you are loading your truck where you are near these tires max something on the truck will likely break before the tires.

Again, I'm not saying you can't or that there aren't advantages. I'm just saying you don't have to if you don't want to or don't have a means to ie no air compressor.

You aren't anywhere near the tires max where heat buildup would be a problem.

Biggles

BassAckwards : you are correct about my tire size and GVWR .

Maybe I will go to 40 PSI to reduce the sidewall flex.The temperature is barely reaching the freezing mark here at the moment,so heat should not be an issue.
The TT will be empty.

Thank you for all the replies so far !

BassAckwards

Also when considering ratings on tires when it says it will do X when X parameters are followed. That means it will do AT LEAST that. So if it says it will do 104 mph at max psi, that doesn't mean if you reach 104.1 mph or even 110 mph for a few seconds to pass someone that the tire will instantly blow out due to the extra heat build up. What it does mean however is that if you are driving at 110 mph for miles upon miles is that it will eventually blow out.

Same thing as load capacity. If you have 2600lbs on each tire at 35psi it will do it without any ill effects. That's what it's rated for. They have to leave a margin of safety and the rating is well within that.

If its rated at 2600 lbs @35 psi and you have 2500lbs on it and it blows out due to heat build up well that wouldn't make it 2600 lbs rated now would it?

Also with max psi. It states it will hold X Psi Cold. That means it will also handle the pressure increase due to heat buildup. That is all factored in.