Overall Diameter versus revolutions per mile

[wrcsixeight]

I've been TireRack tire shopping. Recently, the tire I was leaning toward(Firestone Destination AT) has fallen to number 2, behind the Kumho Road Venture SAT KL61.

This alone is certainly not enough to change my mind, but what I find a little baffling is that the Specs for the FS tire say the overall diameter is larger than the Kumho, but the Kumho states less revolutions per mile.

It is this way in both the tire sizes I am considering.
The Firestone 28.9 diameter LT235/75/15 turns 723 RPmile
The Kumho 28.6 diameter LT235/75/15 turns 706 RPmile
------
The Firestone 29.5" diameter 30/9.5/15 turns 706 RPmile
The Kumho 29.3" diameter 30/9.5/15 turns 654 RPmile

The reviews on both tires are excellent, the only complaints on the Kumho are reduced MPG. I'm wondering if the MPG is just being thrown off because the tires are somehow turning less revolutions, due to Firmer sidewalls?

Tirerack states:
Quote:
The revolutions per mile indicates the number of times the tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from a calculation based on previous test experience.


So do you think one tire was measure in a labratory, the other from previous test results, or do you think the Kumho compresses less under load, and does indeed turn less revolutions per mile, explaining the poorer MPG.

Or do you think the lower MPG reports are due to increased rolling and wind resistance?

If you have any experience with either of these 2 tires you can give your opinion too.

Thanks

 

[CapriRacer]

Originally Posted By: wrcsixeight
.........but what I find a little baffling is that the Specs for the FS tire say the overall diameter is larger than the Kumho, but the Kumho states less revolutions per mile.....


First, tire calculators are available on the internet and those will help you sort this out - EXCEPT:

Every tire calculator I have encountered has a built in problem. They assume that the diameter of a tire directly results in a rolling diameter - and that is incorrect. Typically, the rolling diameter is 97% of the freestanding diameter - and this is pretty close regardless of who manufactures the tire.

BUT - there is one exception to this:

http://www.1010tires.com/TireSizeCalculator.asp

These guys asked for some help when they were redoing their website and I pointed this out to them. This is the only website that correctly factors in this 3% difference. Unfortunately, they did NOT change the width to "Section Width".

So if I use the www.1010tires.com's calculator, I come up with values that are reasonably close to Firestones. It appears that Kumho's are inconsistent.


Originally Posted By: wrcsixeight
.....I'm wondering if the MPG is just being thrown off because the tires are somehow turning less revolutions, due to Firmer sidewalls?.......


Ah....so the real problem is fuel economy.


1) All other things being equal, new tires will consume more fuel than worn out tires. This is due to the additional mass in the tread area for new tires.

2) If OE tires are replaced by replacement market tires, then therer's another hit. OE tires are generally designed with fuel economy in mind, where replacement market tires generally are not.

3) There are other factors that will affect fuel consumption in tires: All Terran tires will be worse than All Season (more tread mass). Tires with high treadwear rating will be worse. (Treadwear / traction / fuel economy tradeoffs)

Since I don't know where you started from, I can't tell you more than generalities - but, NO!, I don't think the differences in diameters are playing much of a role here. After all the differences you listed are only about 1% different. Compare this to a 60% difference that can be experienced just by changing tires (within the same size!)

Oh, and sidewall stiffness plays almost no role in rolling resistance.

Originally Posted By: wrcsixeight
.....Tirerack states:
Quote:
The revolutions per mile indicates the number of times the tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from a calculation based on previous test experience.
.......


Given that a tire manufacturer may have literally thousands of different combinations, I would hazard a guess that hardly any of the quoted RPM values are from actual measurements.

Originally Posted By: wrcsixeight
.....So do you think one tire was measure in a labratory, the other from previous test results, or do you think the Kumho compresses less under load, and does indeed turn less revolutions per mile, explaining the poorer MPG.

Or do you think the lower MPG reports are due to increased rolling and wind resistance?

If you have any experience with either of these 2 tires you can give your opinion too.


So tell me what you replaced with what and I'll try to fill in more detail.

BTW, this time of year, the company I work for gets a ton of complaints about loss of fuel economy. I know that some of this is from changes to a winter fuel mixture and some is to the factors I mentioned above. Interestingly, while we do get complaints the rest of the year, November seems to get 3 or 4 times the number.

 

[wrcsixeight]

Thank you for the Info.

I'm still a ways from being able to afford new tires, and am just researching them at this point.

Of my current Stock sized LT 235/75/15 Michelin xcx/apts

3 were made in the 37th week of 2002, one in the 44th week. They are on 15 x 6.5 inch rims. While they have 65k+ miles on them, they also still have a lot of tread left. The outer sidewalls are badly cracked, and I've noticed some on the inside too. Basically I've been avoiding any high speed driving on them.

About the MPG, as of late with my around town driving, the MPG varies greatly depending on how much the traffic lights like to mess with me. I used to do a lot of distance highway driving and was able to ekk out respectable mileage from my leaden Barn door. I'd keep careful records and try to fill the tank to the same level each fill up, and drive conservatively for ~32 gallons worth.

Basically the Highway MPG averages over 15.5 mpg and I would be very irritated to find that drop because of new tires. At the same time there are times when I will need an AT tire, and am not willing to go all season, Because having a tire fail or getting stuck nullifies any saving from a higher MPG Tire. I have in the past and plan on in the future, to take this campervan to places where people with 4x4's ask how in the heck did I make it there. Offroad traction and durability are important to me, and in these respects my current Michelins have been impressive.

On Tirerack the Firestones:

have hundreds of reviews and I don't remember any saying they noticed any MPG loss.

The Kumhos:

only have about 50 reviews and at least 4 reviewers mention MPG loss.

That said I would also like a slightly bigger/taller tire. According to the radar screens, and highway milemarkers, my Speedo runs about 2 to 3 mph fast at 60 mph. So my MPG numbers are off too. I know I would lose a little accelleration and braking, but perhaps gain a little actual highway mileage and offroad capability.

Interesting observations about November's MPG complaints.

Thanks for sharing your knowledge.

 

[CapriRacer]

A couple of more thoughts:

* - I can't tell for sure, but it appears your van originally came with P metric tires and you are using LT metric tires. All other things being equal, LT's will not deliver as good fuel economy as P metric tires. LT tires are built to withstand a higher unit loading and that makes them higher hysteresis (Higher Rolling Resistance). So you may want to consider using P metric tires in place of the LT's.

* - Inflation pressure: LT's take 15 psi more to carry the same load the P metrics. Have you factored that in?

* - The Firestone's have UTQG ratings of 400 or 460/A/B where the Kumho's are rated 640/A/B. It's no wonder there are fuel economy complaints, the Kumho's have a 50% better treadwear rating. In some respects, the comparison is invalid.

 

[FZ1]

Much adieu about nuthin'. Buy the Firestones. They are better rated and have a better warranty. Buy em at DT and take em back within 30 days to exchange for another tire,if you wish. Free lifetime rotations to maintain you warranty. Now what's so hard about that? Man up,do some chores,and watch some Football. Have a good day.

 

[wrcsixeight]

DT does not sell Firestones.

My Door placard does call for p235/75/15 XL. When I bought it it had Standard load tires on it.

The maximum PSI for both the P Metric and the LT tires in my sizes is 50 PSI.

If a new set of LT tires gives me the same highway MPG as my current LT tires I will be happy.

My Van is a high top camperVan that is currently loaded within 1400 Lbs of it's 6400 GVRW and will be within 500 when I take it to the areas in which I will need an A/T tire.

The Firestones are at the top of my list.

I do not understand the co relation between a higher treadwear rating and higher rolling resistance.

 

[Ken2]

CapriRacer

Is there much difference in fuel consumption between tires of the same nominal size and similar characteristics?

How does a tire maker make a tire to achieve less fuel consumption?

How does a tire maker make a tire that will cause higher fuel consumption, maybe to cut manufacturing costs? (All this is assuming similar tire types.)

Many thanks for your always good info.


WRC
My local Discount Tire store will get any brand in any warehouse in the country, except for those with exclusive marketing agreements or private label brands or lines. Ask DT to beat the TireRack price including shipping. By the way, my Firestone Destination AT tires have given me excellent service, long wear (over 60k and still safe), good fuel mileage, good wet traction, but getting kind'a harsh after several years. (I got these at a Firestone store during a 4-for-price-of-3 sale & 10% off if I opened an account.)

"My Van is a high top camperVan" Yep, you need the tires with the stiffest sidewalls you can find--added stability.

 

[CapriRacer]

Originally Posted By: wrcsixeight
.........I do not understand the co relation between a higher treadwear rating and higher rolling resistance.....


I go into detail here:

http://www.barrystiretech.com/rrandfe.html

Originally Posted By: Ken2
....Is there much difference in fuel consumption between tires of the same nominal size and similar characteristics?....


Well, it all depends on what you mean by "similar characteristics". If you mean to include the same UTQG Treadwear/Traction/Temperature ratings, then the differences would be minor. But that's not usually what folks mean.

Originally Posted By: Ken2
....How does a tire maker make a tire to achieve less fuel consumption?.....


By minimizing the mass of the tire - especially mass in the tread area - AND - by selecting a low hysteresis tread compound - one that does not generate a lot of internal friction due to movement. Unfortunately, that comes at the expense of traction and/or treadwear.

Originally Posted By: Ken2
....How does a tire maker make a tire that will cause higher fuel consumption, maybe to cut manufacturing costs? (All this is assuming similar tire types.)......


While the cost of manufacturing a good low hysteresis tread compound is actually a bit more expense - and the cost is mostly in the material being used - relatively speaking, the additional cost is minor compared to the casing itself. The biggest difference is how much emphasis is placed on treadwear and traction. As we all know, most consumers are only interested in wear, so this tends to dominate what is made available.

But contrast, vehicle maufacturers are especially interested in rolling resistance as it affects their CAFE requirements. So they tend to go the opposite direction from the general public - and occasionally they over-do it and the result is poor traction and/or poor treadwear.

 

[wrcsixeight]

Capriracer, once again thank you for sharing your knowledge.

Perhaps I am making buying new tires more difficult than need be, but I like to be informed about any product I need to buy, because I cannot easily trust salespeople. I want to walk into a tire shop say gimme this tire, and if they do not have it/ cannot get it, I will not let them talk me into something which will most help their bottom line.

I realize that I am looking at an all terrain tire, even though It will only seldom see Mud, dirt, snow and high speed washboard, but I want a tire that can handle those conditions. When This Van was relatively new to me I spent 3 months travelling Baja with 2 spares, and went through more than a half dozen tires. 2 of the tires I had little faith in gave no problems, but the other ones simply could not handle the conditions and I was dealing with a lot of sidewall blowouts or tread separation. More than an inconvenience, a couple times this put me in significant danger. The high speed blowouts are scary enough, but the locations where they occur, and the possibility of lowlifes trying to take advantage is something else entirely.

I am not sure if my door placard was placed on my door before the van got sent to the Conversion company who added windows and added the raised fiberglass roof. While the p/XL tires are recommended on the Placard, the placard was also placed there 22 years ago, and I wonder what changes might have occurred in the tire industry since they placed those recommendations.

SO I'm still trying to decide whether to go p235 LT's or p235 XL's or possibly go a little bigger to 30 x 9.5's.

The most important factors are in order
1. Offroad durability and traction
2. Highway stability/cornering
3. Price
4. On road comfort/Noise
5. Long treadwear
6. MPG
7. Appearance

My current Michelins have met all these well. The only complaint is they do not really absorb road bumps, like lane reflectors, very well. This has gotten worse as the tires age. At 65k+ miles they also still have over half their tread left, so this might indicate that I take turns slowly, and do not turn my wheels lock to lock without the vehicle in motion. I avoid jackrabbit starts, and only brake hard when forced to do so. I've only rotated them when it has been convenient for me to do so. They are all wearing evenly. They are LT tires, they have no treadwear warranty anyway.

But they have taken some serious offroad abuse such as high speed washboard, and 4 wheel drifts over loose/sharp gravel and besides one screw in the tire, no punctures.

I would buy the same brand model again, but they do not make the XCX-APTs any more. The remaining available Michelin lineup is rated lower than and are more expensive than the Firestones.

While I now understand any new tire will have more rolling resistance than a worn out tire, the reports of Significant MPG losses with the Kumhos is what caused me to start this thread.

So Capri Racer, what would you recommend for my intended usage. 235/75/XL's 235 LT's, or 30 x 9.5s?

 

[CapriRacer]

Originally Posted By: wrcsixeight
........While the p/XL tires are recommended on the Placard, the placard was also placed there 22 years ago, and I wonder what changes might have occurred in the tire industry since they placed those recommendations........


Actually the vehicle manufacturer places the placard on the vehicle - and in theory, the conversion company looks at the vehicle specs, making sure their end product is within the limitations of the vehicle. If that is so, then the tires specified on the tire placard ought to be good as well.

But there was a significant event about 10 years ago - the Firestone recall. It was a messy affair, with lots of incriminations being thrown back and forth. but the good news is that this area we are discussing has been improved. In particular, vehicles get larger tires than before. (meaning more load carrying capacity)

Nevertheless, unless you are using Load Range D's or E's, and are using pressures above 50 psi, then you aren't taking advantage of the increased load carrying capacity.

Originally Posted By: wrcsixeight
........SO I'm still trying to decide whether to go p235 LT's or p235 XL's or possibly go a little bigger to 30 x 9.5's......


From a load carrying capacity perspective, 30X9.50R15LT's Load Range C's are the same as LT235/75R15 LR C's. So unless you get Load Range D's or E's, there isn't much point in a change.

Originally Posted By: wrcsixeight
........So what would you recommend for my intended usage. 235/75/XL's 235 LT's, or 30 x 9.5s?


First, I am not one to recommend changes to something that appears to be working - and that's the case here.

Ideally, I'd recommend you oversize your tires taking advantage of the reduced inflation pressure requirement for impact resistance. Unfortunately, the tire industry has moved on, and there are very few on-off road tires available in a larger P metric.

There are some available in 31x10.50R15 - and that is where I would recommend you go. Use 41 psi - the same as the XL P metric.

 

[FZ1]

Different strokes. I prefer to stay with the oem spec tire size,particulary,on a top heavy vehicle like your van. Tires are part of your suspension system.

 

[wrcsixeight]

Thanks again. I thought about 31's, but my rims are only 6.5 wide, and I think they need to be a minimum of 7 inches for 31x10.5x15.
The 30x9.5x15 is as big as I can go.

I cannot afford new rims, but If I could I would go upto 16's and take advantage of the d and e rated tires for those rims.

But as you said, stick with what works. My current C's aren't overloaded

 

[OtisBlkR1]

wrcsixeight : let me save you alot of time and grief, the destinations are friggin awesome, two sets and loved both , my tire/RIM package ive most recently ordered came with Kumho's.. now mind you im comparing apples to oranges becouse i went from a 29' to a 33' tall tire and a 275 to a 325.. so its not even in the same ball park, BUT.. both tires are A/T and both offered outstanding traction. Id have NO problem recomending either.. two weeks after you purchase, im sure you wont be loosing any sleep in hindsight of your purchase..

 

[wrcsixeight]

Thanks for the positive review.

My head is swimming from all the conflicting reviews I've been reading, and PM's, and responses on other forums.

I did find my current tires were made for Sears by Michelin, and were known for their offroad durability and good highway manners, which has also been my experience with them.

They are no longer made.

I will be flying to the other side of the country for several weeks, so I get to think about it for a while longer.