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"  DRIVE GREEN SPECIAL /// ALL-NEW ALT-FUEL TESTS! * CITIZEN FUEL CELL: GM Hydrogen Car Hits 40+ MPG in Long-Term Test  * MECHANIC&amp;#039;S DIARY: Water-Powered Mod Still Can&amp;#039;t Improve Economy  * FIRST DRIVE: Nissan Cube EV-02 Could Hit 150-Mile Range by `10  * TEST DRIVE: BMW 745d Brings 25 MPGs of Luxury Thrust to U.S.  * EURO TEST: With Diesel, Does U.S. Want Jag XF&amp;#039;s `Fright Mask&amp;#039; Look?   Americans burned 142 billion gal. of gas last year. Sen. Barack Obama now suggests that we can trim that thirst by making sure we keep our tires properly inflated. Is he right? Let&amp;#039;s do the math: A single tire that&amp;#039;s underinflated by 10 psi costs about 3.3 percent of your fuel economy. So to keep the numbers round for the sake of our discussion, let&amp;#039;s say four tires underinflated by that same amount would increase our fuel consumption by around 10 percent. How many cars have underinflated tires? A Department of Transportation study dating back to 2001 says that 60 to 80 percent of cars on the road are running tires underinflated by as much as 10 percent. Worse yet, they say that 20 to 50 percent of them are driving with tires down in pressure by as much as 20 percent. Want more? Well, 10 to 30 percent of these cars have tires with pressure as low as 30 percent of the recommended pressure. That&amp;#039;s bad, folks. And it means we&amp;#039;re costing ourselves much more than a few miles per gallon. It means we&amp;#039;re wearing out a lot of tires prematurely. And more important, it means there are quite a few cars on the road that have less-than-optimal control on wet pavement, under heavy braking or during evasive maneuvers. So underinflated tires may be causing untold accidents. So, you say, that study is eight years old! Don&amp;#039;t new cars have tire-pressure monitoring systems (TPMS) mandated by federal law, intended to keep this sort of neglect to a minimum? Well, yes, but only some vehicles with TPMS actually measure the tire pressures with some sort of sender mounted in the tire or on the valve stem. These units tend to be more expensive, and come installed on pricier cars. Most vehicles just check tire pressure with software. Specifically, they use the vehicle&amp;#039;s onboard computer--or one of the onboard computers (most cars have several now)--o provide data through the wheel-speed sensors from the ABS system. Any tire that&amp;#039;s low is smaller in diameter (at least on the bottom) than the others, and rotates more rapidly than its partners. The computer tracks this and turns on the TPMS warning light. There&amp;#039;s an inherent problem with the system. Well, several, actually. The TPMS warning light is only required to announce low pressure when one tire is 20 percent lower than the rest. And that&amp;#039;s far too low to really address the tire wear or vehicle instability issues. Think about it: If all four tires are some percentage less than 20 percent underinflated, there will be no warning--ever.  Here&amp;#039;s the scenario: You stop to gas up and fill the tires on the way home from work one afternoon. Peachy. All four tires are inflated to the pressures listed on the placard pasted to the driver&amp;#039;s door frame. Four months later, you load up the car for a holiday trip. That last time you filled up the tires after work, they were warm, assuming a 20-minute drive from your office to the service station. When they cooled off, the pressure dropped several psi thanks to nothing more than escaping heat. So let&amp;#039;s say the outside air temperature has dropped 40 F in the intervening months. That would lower the pressure by another 5 psi. Even the best tubeless tires lose 2 to 5 percent of their air pressure per month from air migrating through the rubber. Your tires are now somewhere between 15 to 30 percent too low, costing you gas and endangering your family. As we&amp;#039;ve said, the lion&amp;#039;s share of these systems aren&amp;#039;t required to toggle on the warning light until the pressures dip by 20 percent or more--even the ones that actually measure pressure instead of inferring it from ABS data. The average consumer could improve gas mileage by 3.3 percent by simply keeping his tires inflated to the proper pressure. For the average driver in the U.S. and his 15-gal. fuel tank, that&amp;#039;s a savings of about $2.00 on every fill-up. Figure in the increased tire life from those correct pressures, and this is beginning to add up to a handy sum. Of course, if you--or your mechanic--have been diligent about keeping tire pressures set correctly, you won&amp;#039;t save anything, which sounds like rewarding lazy people and penalizing the careful ones to me. But that&amp;#039;s life. Will maintaining proper tire pressures make a huge difference in the enormous amount of oil we import? No. But it can make a dent, albeit a very small one. According to the Department of Energy, underinflated tires alone cost the country more than 1.25 billion gal. of gasoline annually--roughly 1 percent of the total consumption of 142 billion gal. According to the Annual Energy Outlook 2007, published by the Energy Information Administration, offshore drilling would increase domestic production of crude oil by only about 1 percent.  We opened this discussion with Sen. Obama&amp;#039;s assertion that we can offset the need to reopen offshore drilling--and save money at the pump--by keeping our tires inflated properly. He&amp;#039;s right, although he&amp;#039;s ignoring the potential for making a serious dent in natural gas production rates. You know what? I&amp;#039;m not running straight out to buy luxury items with the savings like I was supposed to with my incentive check. But then again, if you&amp;#039;re reading Popular Mechanics, you already know that proper auto maintenance saves you money in the long run.  RELATED STORIES * EARLIER: Can You Really Pump Up Fuel Economy With Tires?  * INTERACTIVE MAP: Worldwide Oil Supplies, Onshore and Off  * PM NEWS: Mechanics Says Obama&amp;#039;s Plane Could Have &amp;#039;Lost Control&amp;#039;  * GEEK THE VOTE: Your Source for Sci/Tech News in the Election   Democratic presidential candidate Sen. Barack Obama speaks at a Missouri campaign stop on July 30. (Photograph by Dilip Vishwanat/Getty Images) "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/cars/how-to/a12139/4276844/>