LT vs P-Metric

acadianbob · 03-19-2010, 11:29 AM

I got into a friend's F150 (virutally identical to mine) the other day and immediately noticed a "firmer" ride. Since the trucks are essentially the same, I asked whether he had LT tires on the truck. Answer was yes. So, an immediately noticable difference in ride quality; as in not as good as the P-Metrics on my truck. I have been thinking of changing to LTs when my Rugged Trails wear out (50M now and expect at least 60M miles). Those of you who have made this switch, do you notice better stability when towing? That would be my motivation for switching. We are towing a TT that weighs just over 5,000 pounds and a tongue weight of maybe 550. Would you make the change from P to LT?

acadianbob · 03-19-2010, 11:48 AM

And here is another thought? What about Load Range C rather than E? It looks to be perhaps a compromise in the middle between P-Metric and Load Range E.

NWJeeper · 03-19-2010, 12:08 PM

Quote:

Originally Posted by acadianbob

And here is another thought? What about Load Range C rather than E? It looks to be perhaps a compromise in the middle between P-Metric and Load Range E.

Bob, P-metric refers to the tire size not the load rating. If you are running Range C now and go up to D or E you will probably notice a big difference in handling while towing, and I mean better. Yeah the ride is stiffer when running around town. My friend at work made the switch on his Chevy truck for towing his boat and couldn't belive the improved handling.

acadianbob · 03-19-2010, 03:11 PM

Jeeper, I think you answered my question about the benefit. I have P275/65R18 SL's on there right now; only rated to 35 PSI. LT275/65R18 Load Range C would give me 50 PSI. And LT275/65R18 Load Range E would go up to 80 PSI. It seems that either of the LT's are likely to provide more stability under towing conditions because of the stiffer sidewall and increased pressure. Thanks for the feedback on your friends truck.

acadianbob · 03-19-2010, 03:15 PM

So, Jeeper, you are right in that I mistakenly equated P-Metric with SL load rating (although I think almost all P-Metric tires are SL, some are XL). I really should have been comparing Load Ratings SL, C, and E.

Dave_Monica · 03-19-2010, 04:43 PM

I have also been contemplating changing the 275/60/20's that came on my 05 Ram1500. The load rating on these is 2601 lbs @ 44 psi verses the GAWR of 3900lbs for the truck (133% of the axle rating). I haven't seen a tire load rating chart, but I'm guessing that a 20" LT tire is probably rated aroung 3500 lbs @ 80 psi. At what point does it become overkill?

I was also considering changing out to 17" LT tires and wheels as they're a little smaller diameter which would increase the rear ratio a bit which would be a good thing.

Dave

NWJeeper · 03-19-2010, 06:23 PM

Quote:

Originally Posted by Dave_Monica

I have also been contemplating changing the 275/60/20's that came on my 05 Ram1500. The load rating on these is 2601 lbs @ 44 psi verses the GAWR of 3900lbs for the truck (133% of the axle rating). I haven't seen a tire load rating chart, but I'm guessing that a 20" LT tire is probably rated aroung 3500 lbs @ 80 psi. At what point does it become overkill?

I was also considering changing out to 17" LT tires and wheels as they're a little smaller diameter which would increase the rear ratio a bit which would be a good thing.

Dave

Dave, 17" is rim size not tire size. Your rolling diameter would remain close to the same and therfore would not have much effect as related to gear ratio. What you would be doing is increasing your sidewall height. Personally I am not crazy about anything over 17" rim size on a vehicle especially one used for towing for many reasons. I think you would be much better served with the 17" rims. The 20" tires and rims are made to "look" good not perform as a truck tire.

Bob, Class C rating is really a "car" tire and they have lighter sidewalls for a smoother ride. This is one reason I don't really like SUVs for towing as the designers goal with an SUV is to carry a family in "car like" comfort and to do so they put the lesser load rating tires on, and put shocks and springs with different valving and rates under them than you will find in a truck. Not to say that many SUVs don't have the power but people towing with them really need to make sure they change their tires up to a Class D or even E for the stiffer sidewalls and make sure their shocks and springs are up to the task. In many cases a set of the Firestone air bags can make a big difference, but I digress.

If you move up to a stiffer sidewall be aware that you only need to inflate them to the amount of load so even with a Class E that may be rated for 80lbs you may only need 55 to carry your load and thus you won't have such a harsh ride, you will however still have the stiffer sidewall and carcas which only improves your stability in towing.

With all that said I should probably change the tires on our Liberty for towing our boat now.....

rockwood06 · 03-19-2010, 08:03 PM

If your truck came with load range "C" tires and you are planning to change out to load range "E" Make sure your rims are compatible. load range "E" are rated for 10,000 and are 10 ply tires and have an 80 lbs max pressure rating, the valve steams should be changed as will. I believe at an 80 psi rating they are concidered high pressure. Standard valve steams will not hold up to 80 psi. On 3/4 ton trucks a load range "E" is standard equipment. A load range "E" tire is the best way to go when towing large campers, control is all part of handling...trust me I know....

Dave_Monica · 03-19-2010, 09:18 PM

Quote:

Originally Posted by NWJeeper

Dave, 17" is rim size not tire size.

Jeeper, you're kidding me, right?

The tire circumference of a LT 265/70R17E tire is less than the circumference of the P275/60/20 tires that I have now. It would be the same thing as changing the rear ratio from the current 3.92 to a 4.10 ratio.

I believe that the Ram tow package includes 3.55 ratio with the 17" tires and a 3.92 ratio with the 20" tires. The final ratio, and engine RPM are the same.

By the way, the F250/350's have an option for 275/65SR20E tires. You need something to fill those huge tire openings in the fenders they give you now!

Dave

acadianbob · 03-19-2010, 09:34 PM

I have an Excel spreadsheet that calculates the circumference of a tire. All you have to do is enter the size. I use it when I am changing size/rim on a vehicle to assess the difference in circumference. For example, my VW has 17" summer tires and 16" winter tires. the winter tires are narrower but have a higher profile so the circumference is the same. If anyone wants a copy of this spreadsheet, PM me with your email address and I'll send it to you.

Dave, I agree with Jeeper that 17 or 18 inch tires are much better for load than a 20. Depending on the size and aspect ratio of the 17" tire, you may very well have a smaller circumference with a 17. Be aware that this will affect your speedometer and odometer accuracy. This can often be corrected by purchasing an "electronic tuner" than can adjust and compensate for the difference in diameter.

acadianbob · 03-19-2010, 09:38 PM

Dave, you are exactly right. Just put your 2 different tire sizes into my spreadsheet, the 17" size is -4.2% smaller than your 20". Your rear axle ratio "change" is exactly right too. 3.92 becomes 4.10.

Dave_Monica · 03-19-2010, 09:51 PM

Quote:

Originally Posted by acadianbob

I agree with Jeeper that 17 or 18 inch tires are much better for load than a 20.

Sorry guys, but I can't agree. All of them will have the same load rating within a 100 lbs, but the 20's will have the shorter, stiffer sidewall and will have the largest contact patch. The biggest deterent is the cost!

Dave

acadianbob · 03-19-2010, 10:29 PM

Until you hit a pothole or road hazard and the short sidewall cannot protect your rim. Ruined tire, bent rim, . . . .

Dave_Monica · 03-19-2010, 10:51 PM

Those 20" E rated will still have a 5" sidewall and at 80 psi, the wheel will be safe. A lot more other things may hurt!

Now, as for the wheels with 235/45r18's on our Mazda 6, they would be a little less round!

Dave

grhodes50 · 03-19-2010, 10:57 PM

In Ford's '09 Towing Guide they state, "Reduce GCWR and Maximum Towing Weight by 500 lbs. on models with 18" or 20" wheels."

NWJeeper · 03-19-2010, 10:59 PM

Quote:

Originally Posted by acadianbob

Until you hit a pothole or road hazard and the short sidewall cannot protect your rim. Ruined tire, bent rim, . . . .

You called this one right Bob.

This is a good reason why you will never see 20" rims on offroad vehicles unless they are running 40" tires. You want that sidewall. Also "low profile" rims and tires are worthless on any vehicle that is 4-wheel drive and really negate the 4x4 altogether. I saw lots of 4x4 SUVs in the ditch last winter in the snow(one on our hill in our neighborhood, the only way in or out) with the low profile tires, while other vehicles had no problems.

Also with many of the low profile rims you cannot use tire chains. Forbidden by the manufacurers of the wheels.

I don't see any semis or trucks used for heavy duty use running down the road with low profile tires. There is a reason for that. Besides the cost

NWJeeper · 03-19-2010, 11:02 PM

Quote:

Originally Posted by Dave_Monica

Jeeper, you're kidding me, right?

The tire circumference of a LT 265/70R17E tire is less than the circumference of the P275/60/20 tires that I have now. It would be the same thing as changing the rear ratio from the current 3.92 to a 4.10 ratio.

I believe that the Ram tow package includes 3.55 ratio with the 17" tires and a 3.92 ratio with the 20" tires. The final ratio, and engine RPM are the same.

By the way, the F250/350's have an option for 275/65SR20E tires. You need something to fill those huge tire openings in the fenders they give you now!

Dave

Dave what I was refering to is the "17" and the "20" at the end of the size is the size Rim that the tires fit.... Reguardless of the rim size you need to buy the correct size tire. Yes you can change sizes and that does change the Ratio but it is not preferable to actually changing the gearing as you end up out of the power band of the engine. Depending on which way you go, you either loose gas mileage or power.

It is a HUGE problem with the Jeep crowd who is always trying to stuff bigger tires under their rigs but then are bummed that they have lost so much horse power. They then try stupid stop gap measures to try and get it back like cold air intake kits and re-chiping the computer when what they should be doing is re-gearing the differentials.

rockwood06 · 03-21-2010, 08:12 AM

Why would one change what the manufacture has design the truck to do with the equipment that came on it, why not buy truck with the correct equipment that is needed to do the job.
I bought a 2008 Chevy pick new, it is a heavy duty 1/2 ton rated @ 10,500 lbs. The frame on the truck is nothing like the standard 1/2 ton and nothing like the 3/4 ton it has it's own design.
The truck came equip with:
6.0 gas engine
heavy duty trans (4) speed
4:10 rear
tires: P265/70R17 standard load range, Goodyear Wrangler AT/S - 2,535 lbs
Heavy duty tow package
factory brake controller
At the dealer I question the tires that came on it from the factory, they the dealer call GM and as the question to make sure that it had the correct tires on it for the trucks weight rating, and they are the correct ones.
Now it was suggested to me by the mechanics that when these tires wear out to have load range "E" installed, But they did say to install the same size tire but in a 10 ply. This would give it a stiffer ride but better control.
I must say I have not any problems so far, but I am going to change the tires to load range "E"
As far as large rims and low profile tires, they look ok on Honda's and golf carts but when it comes to a truck that has to do work like towing you could be asking for trouble, the lower the beed of the tire sets to the ground and you hit a pothole the more of a chance the beed will break away from the rim there is no room for the side walls to absorb the shock and very little flexability when you are carrying heavy loads, so something has to give. On a personnal note 20" feet as they are called on the street with low profile tires is called pimped out and that's fine if you like the look. As stated above put the correct tires on, this way you will not change what the truck was design to do......Just my view.

Dave_Monica · 03-21-2010, 09:15 AM

Guys, I'm referring to factory optioned larger tire/wheel packages. Although the aspect ratios are lower (60 or 65 instead to 70) they are also a little wider therefore the sidewall height is about the same as the factory 17" tire. I wasn't talking about larger diameter rims with 35 or 40 series tires on them!

Dave

Palomino_G210 · 03-21-2010, 09:28 AM

The way I see it.

Low profile tire have the advantage that there is less side wall flex per number of plies. That is to say the side to side flex, squirm, of the tire is less than a high profile tire. All this makes the steering feel more connected to the road, more sporty if you will. The low number of plies will make for a smoother ride and a more car like feel and more vulnerable to potholes. All this speaks against stability when towing as we want to reduce the squirm to make the rig track better. Less side to side.

My truck will spend 90% of it's mileage NOT towing so I am happy to have a more car like ride.

Ideally I should have a different set of wheels/tires for towing than for my daily ride. If I were a contractor hauling stuff and navigating construction sites I would rather have "truck" tires. So in summary, I'll keep my 20" wheels and my car tires for the commute and if the fancy strikes me and I have a spare $1000 dollars I'll get my towing wheels & tires in addition to my snows and my summers.

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