To put it mildly, he!! no. I looked at the specs for the Continental tires you listed and they show they are a LR E with a max pressure of 80 and a max load of 2998, which is pretty low for a LR E. I would suggest running them at 55. Normally I would say 50, but the load rating is pretty low. Whatever you do do not go below 50, even on an unloaded truck. You may have to go even higher if heavily loaded or towing.

 

Based on the info I found, I agree with above. You will need to put more air in the smaller tire to carry the same load. The reason is that the air chamber is much larger in the stock tire, so it can carry more load at a given air pressure to the point of its max a/p. The smaller size you listed is rated LRE, so it can carry more weight total, but at a much higher a/p.
To give you an example if your larger tire were an LT LRE version, it would carry much more weight at 80psi than the smaller one, even though they were the same rating. The difference is Air Chamber size.

Also, on the size difference. The smaller tire will give you better bottom end/low speed pulling, because you changing its total gear ratio. So you are traveling less distance for every revolution of the tire. Also you are traveling less distance for every rev of the engine, so you are putting more hp into a smaller distance, which gives you more power. So, at 60mph, you are turning a higher rpm than you were with your taller tire, and you are not traveling as far either. Your speedo is calibrated to the larger tire, so it is going to tell you that all is well and that you are running the same rpm as you did before at 60mph. However, your actual ground speed will be less than what your speedo says. So, in a given day, you will not have covered as many miles as the speedo says you did, therefore, you won't really know what your mileage is, unless you drive the same route everyday, and you know the exact miles and you know how many gallons/ltrs in normally takes. Then you can see the difference. My guess is you should loose mileage with this tire change. However, you might notice a little more grunt when you take off.

 

Sorry, but it has nothing to do with the tire size. It has to do with the tire load range. A P series construction does not require the higher pressure needed by an LT LR C, D, or E. Looking at the max pressure and max load will allow you to calculate out what pressure is need to carry the load of an F150.
As far as size, even if this was a 35" diameter, 12.50" wide tire, and it had the same max pressure and same load carrying ability, I would use 55psi.

 

I guess I am not sure what you are saying. I agreed with you that he needed to put more a/p in the smaller winter tires than the larger Stock tires. The difference is why he needs to, and how much.
I also don't understand how you can determine what pressure he needs to run on this truck based soley on the max a/p and max load of the tire?

Also, your statement that a P series tire does not require the higher pressure needed by other tires is not accurate. Actually, they are not rated to hold the higher a/p because of their construction. Its not that they don't require it, its because they can't, and are not built too.

Lets look at what all these letters mean. P=passenger, this means that the tire is designed for most passenger vehicles that take that size, and are designed to be in a single tire application. Also, they will be 2 or 4 ply rated with an occasional extra load 6ply in a few applications.

LT = Light Truck. This means that this tire is designed mainly to be on Lt duty trucks, IE pickups. Most of them are approved for dual applications as well. The minimum they will be is C rated, but can also be D or E.
C=6ply, D=8ply, E=10, F=12 and so on.
For the record, most tires are now rated by load index which is what the RMA (Rubber Manufactures Assoc) uses as it's standard, not letters

LR is not a tire designation. It means Load Range. So a LRC = load range C and so on.

There are two things that will dictate how much a/p a tire needs for a specific load.
Size, (yes it does matter) and load index or ply.
The load index/ply or LR(load Range) of a tire will tell you the max a/p of a tire and max load. However, most vehicles are not maxed out or even close most of the time. So they need less a/p in them.
IF you have two tires of the same size brand and tread design and one is a LRD and one is an LRE, the one with the E rating will carry more TOTAL load than the D rated tire. The only reason is because the D rated only goes to 65psi and the E goes to 80. If you put the E rated tire down to 65 psi, it will carry the same weight as the D.

In this particular case, we are talking about two different size tires. The smaller one is of a higher load index/load range than the larger stock tire. The smaller tire can carry more total weight than the stock tire because it is rated to have a higher a/p.
However, if you were to look at what these tires can both carry at 35psi, you will find that the smaller winter tire will not carry nearly as much weight as the larger stock tire at 35 psi. It doesn't have anything to do with max a/p or max load. It has to do with what each tire is rated to carry at 35psi. The larger tires carries more weight at a given psi than the smaller one does, plain and simple.
A P275/65R18 would carry around 2095lbs at 35psi, and the LT245/70R17 LRE at 35psi would carry 1690lbs.
Now, if you max out each tire as they are rated, the stock tire is only rated to have 44psi in it, and the winter tire can have 80psi
If you compare the two tires up to the 44 psi, you will find that the stock tire will carry more weight/psi than the smaller tire. Bigger air chamber carries the same load at lower psi.

I agree that he should run a higher psi than the door sticker says. But without looking at a load inflation table for the brand and size of his tires, I couldn't give an exact recomendation, and I don't see how you could.

Running a tire overinflated is not good, just like running one underinflated is not good. The proper a/p is what we are looking for.
You need to go to your brand of tires web site and look at the load inflation table and weigh your truck front and rear axle to determine the weight. Then you can adjust accordingly.
Sorry this is so long, I am trying to explain this so it makes sense.

 

Perhaps F1 or King might clarify the why, but I have noticed that extra flatness appearance on my winters too. It is not just my truck though, it is the case on almost every single vehicle out there. Assuming that the manufacturer recommends the same tire pressure for all four tires, which is the norm, and almost every car or truck out there has the power-train under the hood, with the extreme weight difference between front and rear, you would be foolish to expect the same level of squish. I have often been tempted to add extra pressure to the front to remove the "squish" effect, but have given the manufacturer the benefit of the doubt. I assume it is probable that they know a tad bit more than what my instincts tell me.

Anyone have an explanation as to why the car manufacturers do not have a higher recommended tire pressure for the front, given the extreme weight difference on a normal, unloaded day?

 

Size really does not matter. Just because a tire is larger or smaller does not mean it will need more or less air pressure. You can have 2 tires that are LR E max 80 psi, one is a 31" and the other is a 35". There is a good probability they will need the exact same pressure.

Everything depends upon the max air pressure and load at that pressure.

As far as the tire that the OP asked about, it is a LR E with a max load of 2998 at 80 psi. His tires need to handle about 2000lbs per tire. Yes added up that is more than the weight of the truck, but not more than the GAWR of the front or rear axle. That is why I stated 55psi for that tire. 35 would be way low and very dangerous. 80 would be way high and cause wear at the center of the tire.

As far as comparing P and LT LR E, I could also find you tires that at the same pressure, they would handle the same weight. There are LR E tires that at 80 will handle over 4000lbs, while there are P series that will not handle 2000lbs at their max.
You have to always look at each tire for max pressure and max weight carrying ability to get proper inflation, no matter what size it is.

Also a P series (except extra loads) have a max weight capacity at 35psi, not 44. The extra pressure is only for sidewall support and normally isn't needed.
Also if using a P series on a light truck, whatever the sidewall of the tire states for max weight carrying ability, subtract about 10% and you get the proper load when used on a light truck.

As far as why the manufacturer does not have different pressures for front and rear, they know no one will go and put extra air in when they need to carry a load. You just go load up and go. Personally as I am just about always running empty, I run a lower pressure in the rear, but only by about 2-3psi.

 

My Michelin X-Ice tires are 255-70-R16. They are not "P", they are not "LT". The max pressure stated on the sidewall is 51 psi. When I bought them, I asked Damon at Tire Rack what to run for pressure and he told me to use the door tag pressure. I run them at 35, they ride well, handle well, and are wearing well. They do not look underinflated.

I run the same pressure in my BFG Long Trails and get about the same gas mileage.

 

GLC, those are P rated, but may be extra load tires. I did take a look at the Michelin web site, but could not find that tire listed under any of the X-ice tires. If I remember correctly when Ford uses XL tires as OE they recommend 38psi.

Unfortunately you cannot tell by looks whether a tire is underinflated or not, especially P series tires.

http://www.tirerack.com/tires/tirete...e.jsp?techid=1

Canadianelbow, you did not state what is the max air pressure, max weight, P or LT, and LR of the tires. Your tires may need more pressure.

 

The reason you can't find them on the Michelin site is they are discontinued - I got a good deal on them on a closeout last year. This is what it says on the sidewall:

Michelin Latitude X-Ice
255/70 R 16 111Q M+S Studless
Max load 1060 kg (2403 lbs)
Max press 350 KPa (51 psi)
Tread plies 2 polyester + 2 steel + 1 polyamide
Sidewall plies 2 polyester

My door sticker says 30 front 32 rear for P255/70R16SL. The load rating of mine crosses to a non-P standard load SUV tire, but at 51 psi instead of 44 psi.

Agreed, but I was responding to this:

 

I see by 2003 that Ford had not learned their lessen on tire pressures. That 30/32 sounds low for a P series standard load on an F150. New ones all say 35. Personally I hate P series tires on a pickup. A problem waiting to happen.

PS, there are P series XL with tire pressures of 51. Also some of the European P metrics use 51.

 

I have no complaints at all with my BFG Long Trail summer tires - they are P, and I run 35 in them. They have about 75k or so on them now and still have better than legal tread.

 

From the Product website.... http://www.bridgestonetire.com/produ.../Blizzak_DM-V1

Size P285/70R17
Service Descr. 117R
Load Range
Speed Rating R
Sidewall Styling BL
Article Number 097198
Appr. Rim Width 7.5-9.5
Overall Tire Diameter 32.8
Tread Depth 14
Weight 47


As for the info on the tire itself, it states Max Load 1285kg (2833lbs) @44psi (max pressure)

From it's description, it is obviously a P tire.

It Also lists it as a 2 ply sidewall.

Nowhere does it state it's load range, neither on the web site, or on the tire itself. It is my understanding that it is to be assumed a standard load, unless it is branded as XL or LL. Whatever a standard load is to mean. I'm guessing they are not talking tablespoons.

 

Yep, those are standard load. If you look close at the site, those that are extra load are marked. Definitely want to use at least 35 on those tires. If the 2833 is what is marked on the sidewall, then the real load carrying ability will be about 2550lbs on your truck.

 

Thanks for the info guys. I am running my winter tires at 55 psi now. The truck has its zip back and doesn't feel sluggish. Next year I am selling these tires and buying the same extreme winter contacts just a bit bigger (to match the original stock size). I will take a hit on them for sure but you live and learn. I was wondering how much weight would one put in the box for the winter. I know the extra weight will give me worse fuel economy but I would rather stay on the road for the winter.

 

I have a 4x4, and never add weight. I do get extra weight from the snow in the bed, but that is all I have ever needed. If you do not have a 4x4 or feel you need the extra weight, I would add about 3-400lbs, making sure that it is also secured. If in an accident, I would not want things like concrete blocks or whatever you use to be flying around.
Another thing to help with traction is to start out in 2nd or even 3rd (if a 6 speed). Less torque and better traction.