The candidates have been talking about it for days. At a town hall event in Berea, Ohio, Sen. Barack Obama brought up the subject again, knocking Sen. John McCain for mocking him for advocating proper tire inflation.
"The Republicans are going around -- this is the kind of thing they do, I don't understand it -- they're going around, they're sending, like, little tire gauges, making fun of this idea, as if this is Barack Obama's energy plan," Obama said in the August 5 appearance. "Now, two points: One, they know they're lying about what my energy plan is. But the other thing is they're making fun of a step that every expert says would absolutely reduce our oil consumption by 3 to 4 percent."
We've decided to check that last sentence, which the McCain campaign and a conservative blog brought to our attention. We found that Obama was correct in a prior claim along these lines, when he suggested that inflating tires properly and getting tune-ups could "save all the oil" we could get from drilling in protected areas offshore.
(The McCain campaign and its allies in the conservative blogosphere have attacked that finding from every angle, but we stand by it.)
But this new claim is different. This time Obama didn't talk about what tire inflation and tune-ups "could" do. He said "every expert" was in agreement that tire inflation "would absolutely reduce our oil consumption" by 3 to 4 percent.
We imagine very few experts would say that.
"You can make some back of the envelope calculations to show that number would be impossible to obtain," said Thomas Menzies, a senior program officer at the Transportation Research Board of the National Academies who managed and drafted a study on tires and fuel economy. So let's do that.
To support the claim, Obama's campaign referred us to this brochure from the U.S. Department of Energy. On page 31 it says: "Keep tires properly inflated and aligned to improve your gasoline mileage by around 3.3 percent."
Similarly, this Department of Energy Web page says: "You can improve your gas mileage by around 3.3 percent by keeping your tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by 0.4 percent for every 1 psi drop in pressure of all four tires."
Under-inflation is a severe problem (affecting 80 out of 81 cars according to one survey), but most tires do not suffer from severe under-inflation.
This survey by the National Highway Traffic Safety Administration found that 26 percent of passenger vehicles had at least one tire under-inflated by 25 percent or more, and the average under-inflation of the four tires for those vehicles (not each tire) was 6.8 pounds per square inch.
So if those drivers corrected their tire pressure, their fuel consumption would be 2.7 percent (6.8 x .4) better. That's just the severe under-inflators -- savings would be smaller for the rest of us. So fuel consumption would fall by something less than 2.7 percent if passenger vehicles corrected their tire pressure.
Furthermore, those savings alone would not result in a one-to-one reduction in oil consumption, as Obama suggested, since just 70 percent of the oil we consume goes to vehicles. The rest gets used in a vast array of products from petroleum jelly to fertilizer.
Also, commercial vehicles have less of a problem with under-inflation, because truckers check their tire pressure religiously to minimize fuel consumption. So they would further drag down the average savings from correcting tire pressure.
Obama's prior claim on this subject was safely couched in generalities and possibilities. But this time he veered into certainties and absolutes -- "every expert," "would absolutely" -- put an exact number on fuel savings, and linked it to overall oil consumption in a way that did not stand up to scrutiny. We find his claim to be False.