LT vs P rated tires on trucks and treadwear?

[buck91]

I spoke with Cooper tech support while shopping for tires and deciding between the Mastercraft AXT and the Discoverer AT3 in either LT265/70r127 or P265/70r17. They told me plain and simple that other than the load ratings and the tread depth the rubber compounds are the exact same between P metric and LT in those models. The compound does differ slightly between the two models, with the AXT having a softer compound, though.

 

[Digital2k2]

Surprised I didn't get more responses about tread wear.

Put a P265/75R16 on a 7" wide wheel and get excessive center tread wear even down at 30psi with an empty bed. Will an LT265/75R16 At it's required higher pressure, say 45psi, wear more evenly due to it's stiffer innards? Or is most of the extra reinforcement on the sidewall and not below the tread?

 

[SteveSRT8]

Originally Posted By: mrsilv04
CapriRacer has explained this P->LT thing so many times that he could probably puke...

And here's yet another thread on it.


Indeed.

And just a note. Sharp attorneys are now using tires as a major piece of leverage in auto accident liability cases. IMO you are nearly crazy to drive on public roads with anything less than a stock tire spec. It can also affect the owners of aftermarket wheels and tires if they can show that they materially affect braking, handling, etc.

 

[CapriRacer]

Originally Posted By: Digital2k2
Surprised I didn't get more responses about tread wear.

Put a P265/75R16 on a 7" wide wheel and get excessive center tread wear even down at 30psi with an empty bed. Will an LT265/75R16 At it's required higher pressure, say 45psi, wear more evenly due to it's stiffer innards? Or is most of the extra reinforcement on the sidewall and not below the tread?


Sorry I didn't respond to your original question, but here goes.

While inflation does have an effect on evenness of treadwear, the affect is small. What is likely driving the center wear is that the tires are on the drive axle. Drive axle tires tend to wear in the center, while steer axle tires tend to wear in the shoulders. Regular rotation evens things out.

Adding inflation pressure tends to slow the wear rate (tire wears more slowly) at the expensive of traction, because the tire is now overall stiffer.

So you would think that LT tires would wear more slowly because they are inflated higher - and that's only partially correct. They also wear more slowly because of the tread ribber compound. On the other hand, they have less traction and consume more fuel.

And here's where it gets difficult: When you compare P type tires with LT type tires, it's hard to get apples to apples comparisons. There are HUGE difference within a given type (P or LT). To get a good comparison, you have to be very careful - and usually, people are making the switch to deal with a particular issue they are having, so they never see a "like to like" comparison.

And one last thing.

Inflation pressure has much more of an affect on treadwear than the construction does. If you've ever rolled a flat tire around, you'll know it is quite soft and floppy (That's a technical term, folks!!) But add some pressure, and it gets quite stiff. Even with just a little pressure, the stiffness goes up a lot - to the point where at normal usage pressure, the construction hardly has any affect.

The other part is that the differences between the construction of LT tires and P type tires isn't as much as you might think. While it takes twice the strength to contain 60 psi than it does 30 psi, getting twice the amount of cord is easy. Polyester fabric comes in a variety of sizes, and to get twice the strength only takes a 40% larger cord (Area of circle sort of thing). That doesn't change the stiffness as rapidly as the increase in strength.

 

[Jarlaxle]

Originally Posted By: SteveSRT8
Originally Posted By: mrsilv04
CapriRacer has explained this P->LT thing so many times that he could probably puke...

And here's yet another thread on it.


Indeed.

And just a note. Sharp attorneys are now using tires as a major piece of leverage in auto accident liability cases. IMO you are nearly crazy to drive on public roads with anything less than a stock tire spec. It can also affect the owners of aftermarket wheels and tires if they can show that they materially affect braking, handling, etc.


On my Dakota...which stock spec? The 195/75R15's the door sticker calls for, the 215/75R15's most 2WD Daks came with, or the 225/70R15's it was sold with.

 

[SteveSRT8]

Originally Posted By: Jarlaxle
Originally Posted By: SteveSRT8
Originally Posted By: mrsilv04
CapriRacer has explained this P->LT thing so many times that he could probably puke...

And here's yet another thread on it.


Indeed.

And just a note. Sharp attorneys are now using tires as a major piece of leverage in auto accident liability cases. IMO you are nearly crazy to drive on public roads with anything less than a stock tire spec. It can also affect the owners of aftermarket wheels and tires if they can show that they materially affect braking, handling, etc.


On my Dakota...which stock spec? The 195/75R15's the door sticker calls for, the 215/75R15's most 2WD Daks came with, or the 225/70R15's it was sold with.



As long as expert testimony can prove that your choice did not reduce your vehicle's ability to avoid an accident.

One of my competitors was sued out of existence running a van with LT tires instead of E's.

 

[CapriRacer]

Originally Posted By: SteveSRT8
As long as expert testimony can prove that your choice did not reduce your vehicle's ability to avoid an accident.

One of my competitors was sued out of existence running a van with LT tires instead of E's.


Ah, a minor quibble, but "E's" are LT's. Did you mean "P's"? If not , then you might want to re-examine the case to see what really went down - to learn from someone else's misery to prevent your own.

Oh and just so everyone understands. Lawsuits are won and lost based on a "preponderance of the evidence" - meaning 50/50 - and the other side will have an expert who will say the exact opposite. So it's a matter of who the jury (or judge) is going to believe. Sometimes people who are correct aren't very convincing. Lots of examples there!

 

[SteveSRT8]

Correction: He did indeed run Pmetrics on a truck that came with "E" tires on it.

When a vehicle is heavily loaded or even overloaded and you use a tire that is not up to the task your liability can be considerable and no expert in the world can save you.

This man lost everything...

 

[Jarlaxle]

He wasn't smart enough to protect himself? Stupidity at work!

 

[yonyon]

I want to feel bad for the guy (even though in the end it probably helped you some) for losing his business over a simple mistake like that but...

It sounds like you have to carry a lot of weight. If he was running a fleet of heavy trucks and decided to cheap out by putting car tires on them he was knowingly risking public safety to fill his bank accounts faster. Was there an accident caused by a tire failure? Were the drivers aware of what they were driving on or were they such people who could be relied upon to know this on their own? Was someone injured or killed?

Did he deserve what he got?

 

[SteveSRT8]

Actually he did do it to try and save money, and if I remember right his tire store got sued too.

Of course there was a terrible accident. Tragic to see an entire business, all its employees and their families, the other folks, man they ALL got hurt. It was hardly a simple thing, and so many people got their lives changed for the worse.

We always buy 3500 series vans that are converted at a Factory Authorized upfitter. GM even puts better springs in ours at the factory for this conversion. No corners are cut. Only the best is used. Even with all this we run perilously close to rated GVWR which is 9700 pounds give or take a bit.

The killer is, these trucks are the cheapest to own in my biz because they are worth much more at selling time. Guys get all cheap on the up front costs and forget the finish line can be 5 or more years away...

 

[kkreit01]

My Ram Outdoorsman came with LT 17" Silent Armor ProGrades. They ride nice, but I don't have anything to compare them to on the same vehicle. All other Ram models come with 17" ,20", or 22" P-metric tires. I will stick with LTs when it comes time to replace.

 

[SwampSurvivor]

Originally Posted By: SteveSRT8
Correction: He did indeed run Pmetrics on a truck that came with "E" tires on it.

When a vehicle is heavily loaded or even overloaded and you use a tire that is not up to the task your liability can be considerable and no expert in the world can save you.

This man lost everything...


That's terrible! It's amazing the impact a decision to save $100 can have on so many people.

How much of this was ambulance chasers being ambulance chasers and how much of it really was caused by running the wrong load rating tire?

There is a 600 pound per tire difference (on average) it seems between Pmetric and LT-E tires in 245-75/16. In 225-75-16 it seems to be about 700 pounds per tire.

Those two sizes seem to be pretty common in both.

What's scary is - how many 15 passenger vans are running around with the wrong tire rating on them? 15 passenger vans are pretty well known for blowing out the LR tire from overloading / under inflation.

 

[Digital2k2]

Originally Posted By: CapriRacer
What is likely driving the center wear is that the tires are on the drive axle. Drive axle tires tend to wear in the center, while steer axle tires tend to wear in the shoulders. Regular rotation evens things out.

Adding inflation pressure tends to slow the wear rate (tire wears more slowly) at the expensive of traction, because the tire is now overall stiffer.



I always thought of it being from such a small amount of weight being on them. Though under hard acceleration I guess they could expand in a way that would put more wear in the center? Saying that I'm picturing drag cars with the big slicks visibly getting taller as they do a burnout haha.

Also, because I have to go a little under 30psi on them for all the tread to make contact has me worried about them overheating. Say you cruise at 80mph for an extended period of time. I'm thinking just because the weight on them is less and they are under inflated to compensate for that, doesn't mean they will hold up to their speed rating as well.

 

[CapriRacer]

Originally Posted By: Digital2k2
I always thought of it being from such a small amount of weight being on them. Though under hard acceleration I guess they could expand in a way that would put more wear in the center? Saying that I'm picturing drag cars with the big slicks visibly getting taller as they do a burnout haha. .....


First, the belts in radial tires tend to reduce the affect inflation pressure has on the footprint shape. Put another way, changes in inflation pressure tend to produce small changes in wear. So if you are getting center wear on the rear tires, it is much more likely that it is being produced by the torque on the tires, rather than the load/inflation.

Drag slicks? Interesting that you should bring that up, but I hope you notice that the increase in diameter is due to speed. Watch a few videos of some runs and notice the tires grow. While you are at it, watch the wave form on the back side of the tire and grow around the the tire during the pass. That wave is the standing wave that is formed in the footprint and the tire trying to rebound to regain the roubnd shape. As the speed increases, there is less time for the tire to rebound, so it doesn't damp out as quickly.

Oh, and you're aware that drag slicks are bias tires, right? They don't have the belts which restrict the circumference. That's why they can grow so much.

Originally Posted By: Digital2k2
.......Also, because I have to go a little under 30psi on them for all the tread to make contact has me worried about them overheating. Say you cruise at 80mph for an extended period of time. I'm thinking just because the weight on them is less and they are under inflated to compensate for that, doesn't mean they will hold up to their speed rating as well.


I don't remember if we discussed what vehicle you have, what tires size you're talking about and what the vehicle tire placard says for tires size and inflation pressure. I re-read the post in this thread, and you've mentioned a P265/75R16 and 30 psi. Are those 2 items on the vehicle tire placard? If not, what are those?

And just to clear up a minor point. Even though there is a difference between the inflation pressure for a P metric tire and an LT metric tire, don't assume that the LT tire (which would require more inflation pressure for the same load) would tend to wear more in the center. The footprint shape can be changed by the tire engineer during the design process to produce a good footprint for a given set of conditions.

And a side note to that is that do NOT assume that evenness of wear is a good indicator of proper inflation. That's why "The Chalk Test" is not a good way to determine inflation pressure.

 

[Digital2k2]

Current tires are stock 265/70R16 (with a max inflation pressure rating of 35psi), however 265/75R16 was an option and I may go to that size next. Both tire sizes have a placard recommended pressure of 35psi front and back. I'm sure the back recommended pressure has to be enough to accommodate the maximum payload rating.

I haven't done the chalk test, but by just laying next to them looking at the edge of the tread on the garage floor, it's easy to tell when the outer edge of the tread isn't making contact. About 29psi in the back and the very outer edge is almost making contact, where the fronts at 35psi are making complete contact.

 

[CapriRacer]

Digital2k2,

I usually do a reality test against a book that lists the tire sizes and inflation pressure by vehicle. If you would be so kind as tell me what the year, make, and model of your vehicle is, I'll look it up. I'm having a little issue with the P265/75R16 being an option.

 

[Jarlaxle]

Wouldn't surprise me, honestly. I once owned a car that had no less than SIX available tire sizes!

 

[Digital2k2]

Originally Posted By: CapriRacer
Digital2k2,

I usually do a reality test against a book that lists the tire sizes and inflation pressure by vehicle. If you would be so kind as tell me what the year, make, and model of your vehicle is, I'll look it up. I'm having a little issue with the P265/75R16 being an option.


My truck is a 2009 Nissan Frontier SE 4x4. The Pro4x package comes with 265/75R16's, bilstein shocks, added lightweight skid plates, a rear locker and different interior material. As far as I know the Pro4x has the same gearing as the SE, and the SE's speedometer is actually more accurate when the 265/75's are put on (many who have changed their tires over claim they have checked with their GPS).

At highway speeds my speedometer does read about 2mph fast, but oddly enough the odometer seems to be accurate to the tenth of a mile even with a 70 mile around trip compared to the GPS.

 

[CapriRacer]

Digital2k2,

My Tire Guides book says that, yes, a 2009 Nissan Frontier Pro-4X came with P265/75R16's, but the book doesn't list any optional tires for that version. Other Frontiers came with other sizes - P265/70R16 is listed - but I would be very careful thinking that they are options for your particular vehicle. Sometimes the weight creeps up on some of these versions and a larger tire size is needed than other versions.

One of the quirks of Tire Guides as a reference is that vehicle manufacturers slice and dice the tire sizing thing in different ways - and it's impossible to tailor the book to reflect each of those. So Tire Guides has chosen a way of displaying the data even if it doesn't accurately reflect what is truly going on.

That said, 35 psi is what is listed for all the 2009 Frontiers, regardless of tire size.