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Psi, Fuel Consumption, and Wild Rides

K. L. Campbell, who works for the Firestone Tire and Rubber Company, makes this claim: By decreasing each radial tire's air pressure from 32 PSI (pounds per square inch) to 24 PSI, the gas consumption of a car traveling at 55 mph would increase about 2%.

What is the impact of this increase? The Department of Energy suggests that if all American drivers kept their tires inflated at the recommended PSI, four million gallons of gasoline would be saved every day!

Question: Do these claims make sense? How could you test them? Also, per the second claim, how many gallons of gas does an individual driver save each day...i.e. how many American drivers are there, etc.?

Note: Given the first stated relationship, you may be wondering if it is linear, etc. That is, if one further lowers the tire pressure below 24 PSI, do related percentage increases in fuel consumption occur...or better yet, does increasing the PSI above 32 PSI lead to less fuel consumption? Unfortunately, the response is: "Once you get above 35 PSI, a diminishing effect occurs. Not only do you not increase fuel efficiency, but you will be rewarded with a harsh and potentially wild ride."

Some related claims: "The Environmental Protection Agency standard that a 1% loss of fuel efficiency occurs for every 2 PSI of air under the maximum level. Add to that the 2003 Department of Energy report that states that vehicles average 22.3 miles per gallon and 12,242 miles per year, and you find that each of the 81 cars burned 144 extra gallons of gas due to under-inflated tires. At $3 per gallon, each car owner is spending $432 for gas each year that they really don't need."

Source: D. Feldman's A World of Imponderables, 1992, pp. 194-195.