Tire "Load Rating" vs MPG 4

[danlindberg]

Hi All,

New here, I was searching for the above info and the discussions here seemed close so...

Does anybody know the relationship between a light truck tire load rating, ie, Load ranges C, D, E, vs vehicle MPG?

Years ago I replaced a set of "P" tires with some "LT" tires and I "think" I remember a decrease in MPG, but this was a long time ago. In that case, it was P at 35 psi to C at 50 psi.

I'm considering upgrading the tires on my light truck but want to know the answer about MPG first.

Thanks,
Dan

 

[evelrod]

I switched from '8 ply rating' Goodyears to '10 ply rated' US Royals on my Dodge duelly, 235 x85R 16's and I saw no change in mpg. I kept the Goodyears at 45 psi and the Royals at 65psi. If there was a difference it was too small for me to see any direct correlation.

Rod

 

[CapriRacer]

Dan,

I don't think there is anything definitive available, so your best bet is to ask folks who have a "feel" for such things - I guess that means yours truly.

Rolling Resistance is more or less a function of:

1) The amount of material in a tire, especially material in the tread area.

2) The amount of movement that material experiences.

3) The resistance this material has to movement.

What this means is that inflation pressure (with no change in load) has a major affect on RR - lower pressure causing more RR.

It also means that a new tire will have more RR than a wornout tire - if the comparison is between the exact same size / design / etc. This is the common comparison, so many folks incorrectly conclude their new tires are inherently less efficient than their old ones.

This also means that a V rated tire will have more RR than the same exact tire in an S rating, because the V rated tire will have cap plies. Of course, you will rarely see V rated and S rated tires that are exactly the same in all other respects.

Tread compound has a major influence in RR. There are 3 things that are traded off: RR / tread wear / grip, especially wet grip. To get good RR, you have to sacrifice one of the others - or both. It is fairly common for OE tires to be designed with RR in mind, which explains why OE tires are frequently faulted for wet traction and wear.

This also comes into play when we talk about tires with high speed ratings. Their tread compounds are designed for grip - so the generally don't wear well, and don't give good RR.

Notice that none of this discussion gets into load ratings. The only way it would enter is if the actual number of plies was different - and typically they aren't.

BTW, RR in tires is a small part of fuel consumption. I've heard the figure 17%, and that seems a bit high. But assuming that is correct, improving the RR 100% would only result in an 8% improvement for the vehicle.

I hope this helps.

 

[danlindberg]

Rod and Capri,

Thanks for the responses. They provide some insite.

Actually, increasing the number of plys is exactly what I'm looking at/for, ie, from a "P" rating, which may or may not be a 4 or 4+2 ply tire to either a D (at 8 plys) or E (at 10 plys).

With max pressures at about 35 psi for the P, to about 60 pis for the D to 80 psi for the E.

And is there increased resistance between the D (8 ply) at 60 psi vs the E (10 ply) at 60 psi, ie, do the extra plys add more resistance even though the tire may be at the same pressure?

Dan

"Notice that none of this discussion gets into load
ratings. The only way it would enter is if the actual number of plies was different - and typically they aren't."

 

[CapriRacer]

Dan,

Allow me to start off by saying:

If your vehicle came with P metric tires, use P metric tires.

If your vehicle came with LT metric tires, use LT metric tires.

Don't switch between the 2 types!

The reason for that is they are designed to a different series of criteria and the vehicles are also designed to accomodate the one type - and more or less not to the other.

Now allow me to correct a misconception regarding "plies".

Pretty much all passenger car tires and light truck tires are steel belted radial tires with polyester plies. Smaller passenger car tires have a single ply, and larger ones will have 2 plies - the crossover point is somewhere around a 215 width, but it varies with aspect ratio and the types of polyester being used - so you will see some differences between manufacturers and even within a given manufacturer.

The important point is that the actual number of plies is not important!!

Belts always come in pairs, and for passenger car and light truck tires, you will only rarely see anything but steel belts. (OK, you truck tire guys will say you've seen 3 belts, but that 3rd belt is really a "protector belt", not a "working belt" and can be removed in retreading without affecting the operation of the tire.)

But there is one last "ply" to talk about - cap plies, sometimes referred to as overlays. This is a circumferential ply which increases the high speed capability of a tire. Typically this is made of nylon. Sometimes more than one is used and nowadays, these are spiraled on, so it is possible to have 2 layers in the center of the tread and 1 everywhere else. BTW, these cap plies only appear in the tread area - just like the belts.

Since it is a US regulation that the construction of the tire be written on the sidewall of a tire, you will see something like this:

Sidewall: 1 ply polyester
Tread: 1 ply polyster, 2 plies steel, 1 ply nylon.

Obviously this varies according to what is in the tire.

One little quirk is that a double spiral wrap of nylon cap in the center of the tread, but only in the center of the tread, doesn't count as 2 plies.

I hope you noticed that I refered to the "body plies" as "plies" and everything else to something different. This is partially because of my background in bias ply tires, but also because I tend to think of belts as augmentation to improve some aspect - just like cap plies do!

One other thing you ought to be aware of:

In the old days, tires were made out of more or less the same material - a square woven cotton of a particular thread size, sort of like canvas. This meant that using more plies resulted in a stronger tire. In fact, tire standards were built around the number of plies, as more plies meant more inflation pressure, which meant more load carrying capacity.

But when stranded threads (not square woven) and synthetic materials started being used, the connection was lost between the number of plies and the strength of a tire (and therefore its load carrying capacity), so the concept of "Ply Rating" was invented. Needless to say this created problems because the actual number of plies was less than the ply rating, so some people thought they were being cheated - which is why the government requires that it be explicited called out on the sidewall.

With the introduction of belted tires, especially radial belted tires, the connection between strength and plies was further confused - and IMHO has reached the point where I don't think the regulation has any value at all!

In order to try to correct the confusion between "plies" and "Ply Rating", some tire standardizing organizations replaced "Ply Rating" with "Load Range" - and that is where we are now!

So when you say "10 ply", you really mean "10 Ply Rating", which is more correctly stated as "Load Range E" - and there is no connection between the number of plies and the "Load Range" - except to say that at some point it is easier for manufacturing to add a ply, rather than to add a rarely used, but stronger material, in the plant.

Whew!! Now to try to answer your question:

"And is there increased resistance between the D (8 ply) at 60 psi vs the E (10 ply) at 60 psi, ie, do the extra plys add more resistance even though the tire may be at the same pressure?"


Ah......Mmmmmmm.......Oh....... OK, I think you can see that the question you asked is almost non-sensical.

But to try to clarify: Load Range D tires are usually 2 plies polyester with 2 steel belts - so are Load Range E tires, except that Load Range E's would have slightly stronger cords - emphasis on "slightly"!!) The stronger cords are typically just more material and therefore would add more mass to the tire and therefore increase the RR for the sae inflation pressure.

BUT!!!!!

The amount of material added is so slight that compared to the things I mentioned in my first posting that you would be hard pressed to actually measure the difference in fuel economy. I am sure we could measure the difference in RR in the lab - but that's why we have labs!!!

Bottomline: Stay with P metric tires - if your vehcle originally came with them. There is no real value in switching to LT metric tires. Further, going to a higher Load Range also doesn't have much value - except to accomodate a higher usage pressure - and that's a whole 'nother discussion!

I know that was a lot and I hope it was useful.

 

[danlindberg]

Capri,

Wow, thanks, just like being back in school.

I think you answered my question, but certainly raised more.

I think the answer is,
There should be little difference in MPG between Load Range P, C, D or E tires. (assuming they all have similar construction layups)

And with this, why are the different Load Ratings offered, why not just put on the highest rated tire and adjust the pressure for the load being carried? (as a owner, not manufacturer who is looking for save many $$ of many units)


I am curious about your comments about not switching from P to LT tires. Why do you say this, as it is commonly done. And maybe even offered as options on some models (I didn't check this but thought I've seen it).

"If your vehicle came with P metric tires, use P metric tires.
If your vehicle came with LT metric tires, use LT metric tires.
Don't switch between the 2 types!"

Note we are talking light trucks and not cars.


This whole discussion is driven by the very low Load Ratings the manufacturers put on their trucks, in my case a 2004 Chev 1500 that has a Load Rating of 1,145 lbs, and after me, the wife and dog are subtracted, only 535 lbs are left. This is both ridiculous and unrealistic for a light truck.

Anyway, I plan to put a light weight camper (about 1500lb wet/loaded) on which is about a 1000 lb overload. (this overloading is also commonly done)

The usual upgrades in the truck camper world are higher capacity tires, helper springs of some nature, stronger (higher damping) shocks and larger sway bars.

Going to a LT tire would increase the tire capacity by 500 to 800 lbs/per tire, and I was curious what the "costs" would be.

Dan

 

[NormPeterson]

Star for that, Capri. Sooner or later, it'll come in handy.


Norm

 

[CapriRacer]

Dan,

Format: I'll quote the question, then answer it.

"There should be little difference in MPG between Load Range P, C, D or E tires. (assuming they all have similar construction layups)"

Yes, except that there are differences.

Let me explain it this way:

Between C, D and E load ranges, there will be differences, but they really won't affect RR.

But P metric tires are different animals. They use lower pressure (assuming the same load) so, they ought to be worse for RR.

BUT!!!!

P metric tires tend to use tread compounds that are good for RR - which I think tends to put them back in a reasonable range - may be even better!

"And with this, why are the different Load Ratings offered, why not just put on the highest rated tire and adjust the pressure for the load being carried? (as a owner, not manufacturer who is looking for save many $$ of many units"

Yes, I agree and I think you will find that with a couple of exceptions that's what happens even for tire manufacturers.

"I am curious about your comments about not switching from P to LT tires. Why do you say this, as it is commonly done. And maybe even offered as options on some models (I didn't check this but thought I've seen it)."

Because these are different animals. I know this is sometimes done (I don't think I would use "commonly" to describe the frequency.)

And I know that some trucks have this as an option.

But LT metric tires require 15 psi more to get the same load carrying capacity. This really increases the spring rate, and a suspension system (springs, shocks, sway bars) set up for lower pressure tires just isn't going to react as well to this higher pressure - and vice versa.

"This whole discussion is driven by the very low Load Ratings the manufacturers put on their trucks, in my case a 2004 Chev 1500 that has a Load Rating of 1,145 lbs, and after me, the wife and dog are subtracted, only 535 lbs are left. This is both ridiculous and unrealistic for a light truck."

I'm not sure I get what you are saying. Is the 1,145 # the payload? If so, you are right, that is unrealistic, but it is what it is, and the vehicle is designed around that. Changing tires to a higher capacity doesn't change the capacity of the rims or the axles or the frame.

"Anyway, I plan to put a light weight camper (about 1500lb wet/loaded) on which is about a 1000 lb overload. (this overloading is also commonly done)"

Commonly done or not, this isn't the way the vehicle was designed. Put a different way, if the vehicle frame cracked, GM wouldn't be at fault.

"The usual upgrades in the truck camper world are higher capacity tires, helper springs of some nature, stronger (higher damping) shocks and larger sway bars."

I'm hoping that you really mean a larger capacity vehicle that also includes all the stuff you mentioned, not a lower capacity vehicle where the frame and axles aren't changed out, too!

"Going to a LT tire would increase the tire capacity by 500 to 800 lbs/per tire, and I was curious what the "costs" would be."

I don't follow what you mean by "costs" - other than what I mentioned.

 

[danlindberg]

Capri,

Again thanks for your responses, lots of good info.


Are these the ones that are "slippery"
The daughtor had a set of new "something" Destinations on her Trailblazer, thes tires would very easily spin in just about any road condition, such that she was almost afraid to drive the car. We replaced them with tires with a much "blockyer" tread and maybe softer rubber, much better.
"P metric tires tend to use tread compounds that are good for RR"


It "seems" common in the light truck world.
"I am curious about your comments about not switching from P to LT tires. I know this is sometimes done (I don't think I would use "commonly" to describe the frequency.)"


Yup, but it is a truck. And probably a larger differnce when going from the 35 psi to 80 psi. That's why I didn't want to go to 80 psi, limiting it to 50-60 instead.
"This really increases the spring rate, and a suspension system (springs, shocks, sway bars) set up for lower pressure tires just isn't going to react as well to this higher pressure - and vice versa."

Yes, I've put more then that in my junk Windstar.
"Is the 1,145 # the payload?"

True, and yes true, that's why I'm trying to learn the capacities off those components also, no point in having 1 piece of the system way larger in capacity. But the tires are what's on the road and gets abused by rocks, logs, holes, etc.
"and the vehicle is designed around that. Changing tires to a higher capacity doesn't change the capacity of the rims or the axles or the frame.


Agreed. Just trying to improve the weak points to avoid future problems.
"Commonly done or not, this isn't the way the vehicle was designed. Put a different way, if the vehicle frame cracked, GM wouldn't be at fault."


No and yes, the truck camper crowd usually goes to the heavier trucks, 250/2500's, 350/3500's and even the 450/550's. But no matter which truck they have, whether 1/2ton, 3/4 or 1+ton, many, (not all) still overload them and make the changes I mentioned. There are folks running up to and over 2000lb overloaded on a 1/2 ton.
"I'm hoping that you really mean a larger capacity vehicle that also includes all the stuff you mentioned, not a lower capacity vehicle where the frame and axles aren't changed out, too!"

You covered it, I was interested in $$ costs, ride/handling costs and anything else that would be caused by switching tires.
"I don't follow what you mean by "costs" - other than what I mentioned."

Dan

 

[CapriRacer]

Dan,

I have been debating how to respond and have decided to break this into 2 parts: 1) Tire related response, 2) Vehicle related response.

Tire Related stuff:

Are these the ones that are "slippery"
The daughtor had a set of new "something" Destinations on her Trailblazer, thes tires would very easily spin in just about any road condition, such that she was almost afraid to drive the car. We replaced them with tires with a much "blockyer" tread and maybe softer rubber, much better.
"P metric tires tend to use tread compounds that are good for RR"

On reflection, I realized that I was trying to explain why vehicle manufacturers would use a P metric tire instead of an LT metric since it seems obvious that an LT metric ought to give better fuel economy due to the higher pressure. There are a couple of things that I neglected.

LT metric tires generally use more tread depth, as well as they use tread compounds that are better suited for higher loading. These 2 things would mean that the tread compound ought to be worse - and maybe much worse - for RR.

It is true that OE tires are generally designed with RR in mind. In the case of General Motors, they generally (the pun is just coincidental!) have the most aggressive RR values and that, of course, means that either wear or traction is sacrificed. I know that GM pays a lot of attention to tire wear, so the result is a tread compound that has some difficiency in the traction department. Plus I have also noticed that traction complaints (and they are never dry traction complaint!) seem to come from specific locales, and there seems to be a coorelation between the tire manufacturer and the locale. I suspect this means that the tread compounding methodologies are different between tire manufacturers (No surprise there!) and it comes out when the tread compound is pushed to an extreme.

I think this is why your daughter had problems - GM's agrressive RR for their OE tires.

But that generally doesn't apply to tire designed for the replacement market. Those tires are designed with wear in mind, and it's RR that gets sacrificed. Any replacement market tire would have been better for traction than the OE tire.

Anyway, I think I may have sung a little off key regarding RR between P metrics and LT metrics.

Vehicle related stuff:

I am really concerned about the rest of your comments concerning what the "camper market" does. Manufacturers spend a lot of time and effort trying to make durable products, and it sounds like the "camper market" takes - oh, we used to call it "factor of safety", I guess it's called something else, now. - and overloads the vehicle / tire / etc. and then doesn't have to pay the price for their modifications.

I guess what I am trying to say is: As much as I know about tires, I know that I don't have a good understanding of what is involved in vehicle design and I am certainly not in a position to over-rule the vehicle engineer when he specifies the payload. I think it is just asking for trouble.

 

[danlindberg]

Capri,

Again, thanks for the info you have provided, it is really helpfull.

Just to be clear, the tires the daughtor has were replacements, but they could have been the original model also, I don't know, as we got the car that way. They were a M+S tread design and "looked" like they would have been Ok.
And in fairness, most of her driving on them was in winter here in MN, ie, some snow and wet.

As for the camper thing, yes, the issue of overweight is a hot topic in the camper world, with folks on both sides. And again in fairness, most try to be within the load ratings, but is so easy to carry more, that most end up overloaded. The hot thing now, on the 250/350 trucks, is to upgrade rims to 19.5 and put on much heavier/higher load rated tires, going from the 3750lb of a 285/16/E to something in the 4600lb range per tire.

As for me, I still am undecided just what I will do, but whatever it is, I want to be comfortable (feel safe) driving down the road.

If you're interested in checking out the truck camper thing, there are a number of sites, the one I read is,

http://www.woodalls.com/cforum/index.cfm/fuseaction/listings/forum/27.cfm

Thanks again for you help,
Dan

 

[tireguru]

Capri, nice answers.

There is no "automatic" relationship between tire load rating and fuel economy. Some tires at 50 PSI have better rolling resistance than other tires at 35. Tread compound and depth choices are moer critical.

There is also no automatic relationship between traction and P/LT metric tires. There are good and poor traction performers in both categories.

In any case, I would never overload a tire. Most 19.5 tires use commercial-grade materials because the tire manufacturers have to include retreadability in their design decisions. If you need the higher load capacity and can afford the greater expense, you should consider that path.

 

[patprimmer]

I would think it is quite straight forward.

If you are really worried about MPG, decrease the load.

If you are more worried about safety, use tyres that are rated for the maximum load and speed likely to ever be encountered.

I'm for the safety option, as the limited load option is unlikely to be strictly followed.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[danlindberg]

As we are looking at a relatively small/light camper, we are still a couple hundred lbs under the stock tire ratings, per tire, and going to the D or E tire would give use some comfort margin.

tire
"In any case, I would never overload a tire. Most 19.5 tires use commercial-grade materials ...If you need the higher load capacity and can afford the greater expense, you should consider that path."



Agreed, unfortunately the 2 brands that make campers lighter also have new price tags %50 higher, ie, from about $11k to $15/16k, and they are not common in our region.
1500lb loaded is on the light side for truck campers.

patp,
"If you are really worried about MPG, decrease the load."

For all you guys, this is a really interested site with good discussions, I'm glad I fould it.

Dan

 

[GregLocock]

FWIW we prepare an information sheet for vehicle modifiers for some of our variants, outlining what they can and can't do load and CG wise and still remain within design parameters. I imagine few pay much attention.


In a modern car or truck you would be well advised to treat the claimed max payload and axle weights as Gospel, there is very little fat in the design that you can rely on.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[danlindberg]

Greg,

Who is "we", and if GM, where does one get this sheet?

Dan

"we prepare an information sheet for vehicle modifiers for some of our variants, outlining what they can and can't do load and CG wise and still remain within design parameters"

 

[GregLocock]

we = an OEM in another country

I don't know how people get hold of it. I imagine they contact the sales people, after all if they are buying a commercially significant number of vehicles from us to modify then they will have reasonable contacts.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.