Dear Cecil: I have a theory that, as a nation, we could save about 10 per cent of our gasoline consumption by the simple expedient of making sure our tires are fully inflated. My questions are: (1) What is the relationship between underinflation and gas mileage deterioration (e.g., 20 percent underinflation reduces mileage 10 percent, etc.)? (2) What is the trade-off between tire inflation and tire life? (3) Do the auto manufacturers in fact specify too low an inflation level for tires in order to soften the ride? Dirk V., Evanston, Illinois
Cecil replies:
This is a deal that can’t be beat. You get to Do the Right Thing and save money at the same time.
The trick in proper tire inflation is to pump in as much air as is safely possible without reducing the traction necessary for acceleration, braking, and handling. Too little air causes your tire to flatten out under load, and the resulting distortion means increased “rolling resistance” (drag) and decreased gas mileage. Too little air also means your tires will be getting an uneven grip on the road, which will cause some portions (typically the outside edges) to wear more quickly than usual, shortening tire life.
A typical tire with a maximum recommended pressure of 32 pound per square inch has a safe pressure range of 24-32 PSI. Variations within that range won’t affect traction that much, but they will affect your gas mileage, ride quality, and tire life. At 24 PSI you’ll get a softer ride but lower mileage and faster tread wear, while at 32 PSI you’ll get a rougher ride but higher mileage and longer tire life. Not surprisingly, in pre-energy crisis days auto makers recommended 24 PSI, but nowadays a lot of them recommend 30 or 32.
Over the years the relentless march of technology has enabled tire manufacturers to significantly improve the gas mileage you can get with their products. Radial tires, for instance, give you about 4 percent better mileage than bias-ply tires because their more rigid construction makes them less likely to flatten out under load. In addition, many of the current generation of tires–the so-called “P-metrics”–are designed to be inflated to as much as 35 PSI, increasing their resistance to distortion. Goodyear makes several lines of all-season radials which can be inflated to 35 PSI and which also use an unusually rigid, nonsticky rubber compound to reduce drag even more. Such tires give you about 2-5 percent better mileage than conventional radials, and about 7-9 percent better than bias-ply tires. Other manufacturers have similar products.
The relation between air pressure and mileage goes something like this: 10 percent underinflation (with 24 PSI being the norm), 5 percent mileage loss; 20 percent under, 15 percent loss; 25 percent under, 20 percent loss. As for pressure/tread life, 15 percent underinflation mean 10 percent loss of tread life; 25 percent under, 20 percent loss; 50 percent under, 40 percent loss. These figures are meant to give you the basic idea and are subject to considerable fluctuation depending on individual conditions.
By the same token, if you increase pressure from 24 to 30 PSI, you’ll increase your gas mileage about 3-4 percent (more in some cases) and your tire life maybe 3-6 percent. As a general rule, the best thing to do is check the maximum safe pressure printed on the side of the tire and maintain inflation at or slightly below that level. (Caution is advised with older tires. For accuracy’s sake, make sure you check pressure when the tire is cold, using your own gauge if possible.) The ride may be a tad on the rocky side, but think how patriotic you’ll feel.
Cecil Adams
Send questions to Cecil via cecil@straightdope.com.