Ultra High Performance Tires(UHP) and Fuel Economy losses?

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I went from an all season T rated touring Goodyear Eagle GA to a wider UHP Z-Rated (W) Michelin Pilot Sport A/S. 235/55/TR16 96T -----> 245/50/ZR16 98W Can this account for losses between 2-3mpg? The handling and wet grip is amazing, but this must come at a cost of higher friction and rolling resistance. I swear since the change my car FI has dipped severly. Again the handling is amazing compared to the snow and rain near deadly GY's.
 
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There are 2 reasons why the type of change you did would adversely affect your fuel economy. 1) Going from worn tires to new tires 2) Going from fuel efficient tires to high grip tires. Rolling resistance is directly coorelated to weight. So a worn out tire will have better fuel economy than a new tire - even if they are identical. High Performance tires have a double whammy for you. The construction is beefier (more weight) and the tread compound is designed for grip and that means sacrificing rolling reisistance and wear. The design you replaced is known for its poor traction, but it is fuel efficient.
 
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In addition to the above, the wider the tire (and you said you went to a wider tire) the more the rolling resistance, because you have more tire on the ground (times 4). Generally speaking, the more aggresive the tread pattern, the more the rolling resistance and the less mpg you get, especially among types of tires (summer tires, all-season, all-terrain, and mud tires), but also, even within a type of tire. For example many all-season tires get higher mpg than other all-seasons.
 
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How long have you been running them? The Pilots would likely have more rolling resistance due to the softer, stickier compound, the more aggressive tread design, the extra width, and the smaller diameter. Being 2% smaller in diameter makes your gearing a little shorter too. What conversion factor are you using?
quote:
Rolling resistance is directly coorelated to weight. So a worn out tire will have better fuel economy than a new tire - even if they are identical.
I think the weight of the worn out tread is insignificant. I'd probably have to put an extra 400lb in my 2800lb car to get a 2mpg difference on the highway.
 
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rpn453, the difference is that the tires are unsprung weight so it's not the same as throwing a few extra pounds in the trunk.
 
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I disagree, QP. Unsprung weight can certainly affect handling and suspension, but not necessarily fuel economy. From Tirerack, the Goodyears weight 25lb each and the Pilots weight 27lb each. So there's 2lb extra per wheel and 8lb extra total. For the purpose of rolling resistance, the force of the tires on the ground increases by a total of 8lb. For the purposes of acceleration or braking, the equivilant mass increases by almost 2 times: 16lb. These numbers are small relative to the mass of the car, which is likely over 3000lb. So, IMHO, the extra tire weight is not a significant factor here. It would contribute a little to the loss of mileage, but I think tread pattern, compound, and width are the bigger factors here. I don't know how much difference there is between a new tire and one with the tread worn down, but I wouldn't think it would be much more than a couple of pounds. [I dont know]
 
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rpn453, agreed. The 2 extra lbs per tire will not account for his 2-3 mpg loss alone. I was just saying, 2 lbs on the rolling tire is not the same as 2 lbs in your trunk.
 
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Guys, You're misunderstand the point about the weight. A tire's ROLLING RESISTANCE is affected by the weight, especially weight in the tread area. Weight loss is not insignificant relative to the weight of the tire, especially weight loss in the tread area. Also, the tread compound has a significant affect on rolling resistance and high grip compounds generally have high rolling resistance. BTW, just because a tire weighs 2 pounds more doesn't mean it has 2 pounds more drag. Drag is a force, weight is mass.
 
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I wondered if that was what you meant, CapriRacer. How is rolling resistance affected by tire weight? Do you have any quantitative data as to how big a factor it is? I can see how a higher-traction tire could have more rolling resistance. The tire has to scrub against the ground as it rolls over it, just to conform to the surface. More traction would cause more resistance to that, and more rolling resistance. Makes sense. Also, wider tires have a surprising affect on aerodynamic drag. I've seen lots of instances where manufacturers quote different cD numbers for regular and sport-tire versions of the cars. Typically it seems the difference is on the order of .02, or around 7%. Aero drag is a very large factor in fuel economy. Still I suspect a difference in driving habits is part of the cause here. - Glenn
 

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Vehicle is a V6 3800 Camaro w/ 4speed automatic. Maybe I have been driving faster. The Pilots give me a huge boost in control and confidence. The T rated cushyness of the touring GY Eagle GA's kept from higher speeds and fast cornering. The GY were quite and had a luxury ride quality. Whereas the MN Pilot A/S are much noiser. I miss the comfort, but I love the Pilots performance especially on wet and cornering. I know performance v comfort are inherently at odds and you can not have it all. Maybe I should replace the factory 02 sensors, plugs & wires and see if this helps at all. Maybe switch to M1 5W30 from GC OW30. Last time I may replace them with GY's just to baseline fuel economy until a change over to Summer UHO tires. Thanks for the responses keep them coming.
 
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"How is rolling resistance affected by tire weight? Do you have any quantitative data as to how big a factor it is?" Try reading Dr. Diesel's Secrets to Great MPG. You can acess the pdf file here: http://www.everytime.cummins.com/every/misc/mpg.jsp While this applies to over the road trucks, there's a lot of really interesting information. But in particular, look at page 19. There is a graph showing the effect tread depth has on fuel economy. By extension, this means that weight in the tread area has an affect on rolling resistance. While I don't have any definitive tests that demonstrate this, compiling a number of unrelated tests seems to point to weight in general, but especially weight in the tread area - even if it is not tread compound - is related to rolling resistance. Certainly, tire engineers designing tires for vehicle manufacturers are aware of this and carefully balance the requirements of the OEM. Sometimes this means a discussion about what the requirements OUGHT to be.
 
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Thanks for the link. I haven't looked at it yet but have it loading in another window. I can see how tread DEPTH would affect rolling resistance. The tread is somewhat flexible and squirms when pressed into the ground by the weight on it every revolution. It then resumes its original shape when it leaves the ground. This energy becomes heat and is wasted, and shows up in rolling resistance. However, it has nothing to do with the weight of the tread - it is the deformation itself, and the energy required to cause the deformation, that is the cause. - Glenn
 
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"....This energy becomes heat and is wasted, and shows up in rolling resistance. However, it has nothing to do with the weight of the tread - it is the deformation itself, and the energy required to cause the deformation, that is the cause....." Not quite. If the tread deflects then returns to the same shape, the net effect is no work is done. So where does the heat come from? It comes from the internal friction of the elements that are moving, and the amount of heat generated is a function of the volume of material that is moving, and the volume can be stated in the form of weight.
 
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Energy is often lost as heat even though, depending on the way you look at it, there is no net work done. Compress a spring a bunch of times and let it return. No net work is done, but some of the energy is lost. Where does it go? Into heat. The work is being done at the molecular level inside the material. More to the point, consider this: fill in the entire void of the tread with rubber of exactly the same consistency. Squirm will go down, and weight will go up. What will rolling resistance do? It will go down. - Glenn
 
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"...The work is being done at the molecular level inside the material." Same thing with the rubber in the tread. I have a tendency to use the mental image of spaghetti strands rubbing against each other and the friction causes the heat. Needless to say, metal is a crystal, so this mental imagine is reserved for rubber. But the principle is the same "....More to the point, consider this: fill in the entire void of the tread with rubber of exactly the same consistency. Squirm will go down, and weight will go up. What will rolling resistance do? It will go down..." Sorry, RR goes up! This was discovered during some of the early attempts to improve rolling resistance. Same thought process, but the result was the opposite of what was expected. Look at it this way. While the squirm would go down, the resistance of the rubber to this movement is relative small compared to the resistance caused by the cord material, so the movement is hardly affected at all by filling in the voids. Today, tire engineers are keenly aware of this phenomenon and make attempts to slim the tire down as much as they can. But increasing the void ratio is going the opposite direction for tread wear - and tires with low rolling resistant compounds don't wear as well - it's a balancing act that takes several tries to get it right.
 
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quote:
Sorry, RR goes up! This was discovered during some of the early attempts to improve rolling resistance. Same thought process, but the result was the opposite of what was expected.
I think that would be due to the larger surface area of the tread, and not necessarily the mass of the tread. I'd like to see that graph on page 15 show the fuel economy when the tread depth reaches zero, or even below zero. That could settle it! Good link, but I've yet to be convinced that the fuel economy gains resulting from tread reduction are due to the reduced mass of the tread. [Smile]
 
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Guys, I know this is a difficult concept to grasp. However, it is an observable phenomenon, but you have to be in a position to observe it. I'm an engineer for a major tire manufacturer. I know at least one of the vehicle manufacturers requires a check on the RR to assure the tires remain within a reasonable range compared to what was qualified. They require this in case the EPA fuel economy numbers are checked by the government. Every quarter some sample tires are pulled from the warehouse and tested. The data is recorded on a control chart. If you look past the scatter of data, the RR values gradually increase over time. As the values approach the control limit, the plant is asked to check on the production. Usually, the extrusion die used to make the tread slab has worn and a new, thinner die has to be made. BTW, we are talking hundredths of an inch in tread thickness and fractions of a pound. To put this in perspective - the molds remain the same and what varies is the amount of rubber between the bottom of the tread groove and the top of the belt. We call this "undertread". In essence, our QA department is conducting the verification experiments on a regular basis.
 
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I think you'll find that the tire's air pressure is the biggest factor in rolling resistance. Many high-performance (V & Z rated) tires perform best with more air than touring-type tires. And indeed, the maximum air pressure printed on the sidewall is uaually higher, too.
 
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