Increase Your Gas Mileage

Below are some common hypermiling suggestions -- and an expert's view on whether the technique is smart and safe. We asked auto expert Pat Goss -- owner of Goss' Garage in Seabrook, commentator for PBS's "Motorweek," and host of a regular chat on washingtonpost.com -- to weigh in on what works.

DRIVE THE SPEED LIMIT

Expert's take : Goss says that as you go above 38 mph in most cars, you lose mileage. For every 5 mph above 55, he says you can lose as much as 10 percent of your fuel economy. So slowing down can save you gas.

DRIVING WITHOUT BRAKES (or in hypermiling lingo, "d.w.b.") is all about coasting. Congestion is constant in Washington, and accelerating from zero to 20, then back to zero, is inefficient. Instead, if the car in front of you is speeding up, maintain a steady speed and let it get ahead of you, when traffic starts to slow back down you'll catch up.

Expert's take: Do it when possible -- but be careful. "You're probably going to have some highly ticked off people if you do it on the Beltway," Goss warns.

TURN OFF YOUR CAR AND COAST , aka the "forced-auto stop." In hybrids, the internal combustion engine shuts off at stops to conserve fuel; the electric batteries keep the car running. To save even more fuel when decelerating, some hypermilers -- including Gerdes -- shift to neutral and turn off the engine while coasting to a stop.

Expert's take: "Highly dangerous. You don't have your car under control," says Goss. (In other words, not all hypermiling techniques are good ones.)

OPTIMIZE YOUR ROUTE : Avoid big hills or stop-and-go traffic. Test different routes to see which is the smoothest ride. Sometimes, a longer route with better driving conditions uses less gas.

Expert's take: "Basic driving techniques. I teach this the first day," Goss says.

WATCH YOUR TIRE PRESSURE . It takes a lot of extra energy to move even slightly flat tires. Some hypermilers recommend over-inflating tires.

Expert's take: Goss says this works but is very risky. "When you over-inflate a tire, you can compromise its traction and . . . make the tire wear out more rapidly," he says, adding that it could be "very negligent to recommend that someone do that."

STAY ON TOP OF OIL CHANGES , and use thinner oil.

Expert's take: "It can have a significant effect on fuel economy, especially as the oil ages," Goss says. "The viscosity of engine oil is always increasing. . . . The thicker the oil is the harder it is to push through the engine."

-- J.Z.