Obama Tire Guage Energy Plan

Barack Obama just suggested that Americans can help conserve energy by inflating their tires. The McCain campaign is making a bit of a joke from this suggestion. In fact, this morning the McCain campaign distributed tire pressure guages to reporers and emailed supporters offering them “Obama Energy Plan” tire pressure guages if they donate $25.

Is this whole tie pressure bit just a lot of hot air? Not really…although it won’t be a major solution to our energy problem, little things do help. Keeping tires inflated properly can improve gas mileage 3-4% resulting in a savings of about twelve cents a gallon, according to the overnment’sd website www.fueleconomy.gov

According to the Energy Information Administration, in the US we consume 9,286,000 barrels/day (390 million gallons/day) of gasoline. Three percent of that consumption is a savings of 11.7 million gallons a day. Here’s some interesting related stats:

Consumers would spend 46.8 million dollars less a day for gas (assuming $4 per gallon price)..so that’s 1.7 billion dollars a year less that consumers would be spending on gas that could be spent elsewhere in the economy! Interesting numbers…the fact is that we’re consuming so much oil that even a minor savings can mean a substantial dollar impact!

No, I think Obama’s point about inflating our tires was presented incorrectly (at least in tone) as a major solution to our problem, but it has a bigger dollar impact than many of us might think! Don’t these dollar numbers surprise you? Of course, to get 100% of Americans to inflate their tires (or do anything) is unrealistic..but even if half of the population did it, the impact could still be helpful.

Low resistance tires

Low Rolling Resistance Tires

Rolling resistance is fundamentally the parasitic energy a tire consumes while rolling under load. The phenomenon is quite complex, and nearly all operating conditions can affect the final outcome. It is estimated that 5%-15% of light-duty fuel consumption is used to overcome rolling resistance for passenger cars. For heavy trucks, this quantity can be as high as 15%-30%.

Some of the following documents are available as Adobe PDFs. Download Adobe Reader.

Sensitivity Coefficient

A modeling study conducted at the National Renewable Energy Laboratory, Analysis of the Fuel Economy Benefit of Drivetrain Hybridization (PDF 268 KB), shows the sensitivity coefficient (percent increase in fuel economy per percent decrease in rolling resistance) to be as high as 0.2 for a conventional vehicle. The current study quantifies this number by duty cycle and comes up with a range of 0.1-0.2. A report written by Green Seal (PDF 285 KB) indicates that a 1.5%-4.5% savings is possible for high rolling resistance tires. The same report quotes testing done for rolling resistance on tires with rolling resistance coefficients varying from ~0.01 all the way down to ~0.006 implying up to 3% improvement in fuel economy for a very low rolling resistance tire.

New vs. Replacement Tires

New cars are generally equipped with low rolling resistance tires that offer better fuel economy. This helps the auto manufacturer to meet their Corporate Average Fuel Economy (CAFE) standards. However, no requirements are currently placed on replacement tires. Therefore, if you want to purchase fuel-efficient replacement tires, you must research to figure out which tires have low rolling resistance.

Tire Inflation

Proper tire inflation also plays a large role in keeping vehicles fuel efficient. For example, a vehicle with a recommended pressure of 35 psi whose tires are at 28 psi will have increased its rolling resistance by 12.5%.

Cost Effectiveness

According to a California Energy Commission report, the use of low rolling resistance tires on light-duty fleets is cost effective over the life of the vehicle. That is, the fuel savings pays for the additional cost of the low rolling resistance tires. This report finds that the opportunity for cost-effective energy savings in California from low rolling resistance tires is substantial—about 300 million gallons of gasoline per year resulting from a 3% average improvement in the fuel efficiency of light-duty vehicles currently operating on replacement tires. For more information visit the California Energy Commission’s Fuel Efficient Tire Proceeding Documents Web page.

In the equation, it is assumed that the baseline rolling resistance is 0.009. RRC_replacement is the rolling resistance of the replacement tire which can be obtained from sources such as Green Seal’s Choose Green Report, March 2003 (PDF 285 KB) Download Adobe Reader. Negative values indicate an increase in fuel consumption.