National Academies Report on Tires and Fuel Economy
04 April 2006
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| Energy flow for a modern mid-size sedan. Rolling resistance consumes about one-third of the mechanical energy output from the engine to the wheels. |
Two divisions of the National Academies—the Transportation Research Board and the Board on Energy and Environmental Systems—have issued a special report examining the contribution of tires to vehicle fuel consumption and the prospects for improving tire energy performance without adversely affecting tire life, traction capability, and retail prices.
Tires and Passenger Vehicle Fuel Economy concludes that (1) reducing the average rolling resistance of replacement tires by a magnitude of 10% is technically and economically feasible and (2) such a reduction promises a 1% to 2% increase in fuel economy—representing between 1 to 2 billion gallons of fuel saved.
Tires affect vehicle fuel economy mainly through rolling resistance. As a tire rolls under the vehicle’s weight, its shape changes repeatedly as it experiences recurring cycles of deformation and recovery.
In the process, mechanical energy otherwise available to turn the wheels is converted into heat and dissipated from the tire. More fuel must be expended to replace this lost energy.
Combinations of differences in tire dimensions, design, materials, and construction features will cause tires to differ in rolling resistance as well as in many other attributes such as traction, handling, noise, wear resistance, and appearance.
Once they are placed in service, tires must be properly maintained to perform as intended with respect to all attributes. The maintenance of proper inflation pressure is especially important.
Although reducing the average rolling resistance of the tires may be feasible technically and economically, the report also concludes that consumer understanding and behavior is a major obstacle.
The committee does not underestimate the challenge of changing consumer preferences and behavior. This could be a difficult undertaking, and it must begin with information concerning the tire’s influence on fuel economy being made widely and readily available to tire buyers and sellers.
A significant and sustained reduction in rolling resistance is difficult to imagine under any circumstances without informed and interested consumers.
The committee observes that consumers now have little, if any, practical way of assessing how tire choices can affect vehicle economy.
The main source of rolling resistance in on-road driving is the repeated flexing (deforming and recovery) of the tires as they roll. Through an effect known as hysteresis, this repeated flexing causes mechanical energy to be converted to heat. This is turn, requires the application of more mechanical energy from the engine to replace the energy lost as heat from hysteresis.
The design, construction, and materials of tires, as well as their maintenance, their condition, and operating conditions—although not an increase or decrease in travel speed—affect the rate of energy loss.
To improve traction and prolong wear, the tread component of the tire must have a substantial portion of the deformable, hysteretic material in the tire. The type and amount of material in the tread are therefore important determinants of rolling resistance. Other tire features and design parameters affect rolling resistance as well, including tire mass, geometry, and construction type.
According to the report, the large majority of new passenger tires, properly inflated, have rolling resistance coefficients (RRC) ranging from 0.007 to 0.014, with most having values closer to the average of about 0.01.
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| The distribution of RRCs across the tires examined for the report. |
The spread in rolling resistance values has increased of late, however, due to proliferating sizes, types and speed capabilities. Tires with high-speed ratings—which have become more popular in the replacement market—tend to have higher-than-average rolling resistance.
The committee concludes that its 10% reduction in average rolling resistance could occur through a combination of changes in the distribution of tires purchased and greater use being made of various technologies (materials and design) to reduce rolling resistance. It could also be achieved in part through more vigilant tire maintenance.
Resources:
It would help if the driver had information about tire pressure on the dashboard, or at least a type of alarm/warning when the pressure falls below a certain level.
I believe such system exists for some commercial vehicles. Would it be that costly to implement in passenger vehicles? Beyond reducing fuel consumption it would also increase road safety and reduce wear on the tires.
Posted by: Jozef | 04 April 2006 at 11:50 AM
Jozef:
Low tire pressure warning devices are now required equipment on all 2007 models.
Posted by: Cervus | 04 April 2006 at 12:04 PM
Maintaining the correct pressure is indeed the most important fuel economy strategy regarding tires. Low-resistance tires have reduced grip, many Prius owners have reportedly exchanged theirs - safety must come first.
Deflation warning system:
(DWS) developed by Dunlop for on-line detection of tire pressure loss. When tire pressure is reduced, the tire circumference is also reduced resulting in increased wheel rpm. The system uses the wheel speed sensors of an existing ABS system to continuously monitor wheel speed and tire condition, and triggers a warning signal upon detecting a problem.
Sumitomo, Continental and others have commercial implementations. DWS is available on some series production vehicles, e.g. Mercedes B-Class and BWM 7 series.
A Finnish company has a retrofit option based on alternative technology:
http://www.finnfacts.com/english/main/actualities/liikenne.htm
Posted by: Rafael Seidl | 04 April 2006 at 12:15 PM
How does a consumer find out the LRR rating for tires at purchase time? This data seems to be closely held by the manufacturers. When I went to buy new tires for my Prius, none of the vendors even knew what rolling resistance was.
Posted by: jeffd | 04 April 2006 at 03:35 PM
jeffd-
It would be nice to know that, but I have given up trying to find the information. The only thing I have come up with is "backdooring" that information. This article claimed that higher speed rating tires have a higher rolling resistance, so there is a clue. I don't need tires rated to 135mph. I would think inclement weather tires might also be worse. I also believe heavier tires are detrimental. The trouble is I still think the most important thing if you are cheap like me, is to find tires that will last. The cost of new tires, tax, balancing, the time to get new tires, etc is, I think, more than you might save in fuel. But maybe you hate buying fuel.
JRod.
Posted by: JRod | 04 April 2006 at 07:43 PM
If cutting tire rolling resistance in half only improves fuel efficiency by 1% then I would look elsewhere for fuel consumption answers. Even an idle-stop hybrid system can provide several times that improvement without compromising braking traction.
Posted by: tom deplume | 05 April 2006 at 09:13 AM
When my original low rolling resistance tires on my 2003 Honda Civic Hybrid wore out, I replaced then with tires that had a good LRR rating, though not as good as the original Bridgestone's. I saw a drop in average fuel economy from 49.5 to 46.5 as a result (and this is after 1 year, so the new tires have been broken in)
The reason for the switch? Safety, I hydroplaned more often than was comfortable with the original tires and price. these replacement tires were about $40 vs. $75 for replacement Bridgestone's.
Posted by: Joe Herman | 05 April 2006 at 12:31 PM
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Posted by: biagra | 01 August 2007 at 07:30 PM
Simple Question: at a given road speed, does an underinflated tire exhibit higher RPM's than an overinflated tire?
Another way to pose the question: Over a fixed measured straight line distance, will and underinflated turn more times to cover that distance than an overinflated tire. (In both cases, assume all 4 tires have same pressure.)And in both cases, is there any personal experience from experimentation?
The final question: using only the odometer and the gas pump meter, can a driver actually and accurately tell if he or she is getting more miles per gallon or less?
Consider this before answering:An underinflated tire will (this is my belief based on heat generated by underinflated tire)cause burning more fuel over a measured distance.....however...if you believe the wheel will spin more ...then there will be more miles exhibited on the odometer..and thus you will not be able to see a less mpg from your calculations!
I will try this this weekend.
Posted by: Rich | 31 October 2007 at 12:20 PM
....forgot to mention. How can a tire with a fixed circumference in the form of fixed tread, turn more RPMS simply because it is distorted from underinflation? In other words, if you were to cut off the tread and lay it end to end repeatedly,it will have fixed number multiples since the length of the tread will not be affected by inflation.
Since the tread is physically attached to the axle via the wheel bead etc, this ratio of layed down circumference to wheel rpms would seem to be a fixed relationship regardless of 25PSI or 35PSI.
Posted by: Rich | 31 October 2007 at 12:25 PM