Conservation buffs, write this on the back of your hand when you go car-shopping: city driving—hybrids; highway driving—diesels. If there's still room, add: Fuel efficiency doesn't mean a Toyota Prius in every circumstance. There are multiple ways to use less fuel. Your choice of fuel-saving technology depends on the kinds of driving you do. In city and suburban driving, where there's a lot of stop-and-go, the ability of a hybrid to regenerate energy from deceleration really boosts efficiency. And it's a substantial amount, more than half of the energy that's otherwise wasted as brake friction and heat. That's why a lightweight hybrid really can get about 50 mpg in city driving.

But once you're on the highway, there's precious little deceleration as a fraction of your total driving time. Engine efficiency matters most, and for that, a diesel engine reigns supreme. Now that we have low-sulfur-fuel, clean-burning diesel engines, they'll become increasingly common in 2008. If you've been to Europe and recall clattering diesel taxis emitting an irritating perfume of diesel exhaust, forget what you know. Current and future diesel passenger vehicles sound, smell, and perform like gasoline-engine vehicles. One exception: On cold mornings, you'll hear some clatter for a minute or two, when Herr Doktor Rudolph Diesel's eponymous invention first starts up.

Here are ten ways to be more efficient. It's not just about the car you buy; it's also how you drive and who you drive with. And the last couple tips are common sense, and have been the staple of AAA and EPA bulletins for more than a generation.

1. Buy a hybrid. Get a hybrid if you're the kind of person who was known in the 1990s as a "soccer mom." Meaning lots of urban and suburban travel, shopping, and mall-hopping. It also means working soccer moms and dads commuting to work, but not on interstates, or on interstates with stop-and-go traffic.

2. Buy a diesel. This is the most efficient way to burn fuel at highway speeds. Wait until 2008, or even 2009, for more alternatives. Your choices right now are Mercedes-Benz and pickup trucks.

3. Buy a variable displacement vehicle. At highway speeds, you're using a quarter of the engine's capabilities; a tiny four-cylinder could power the stoutest SUV. Variable displacement shuts down fuel flow to half the cylinders, but air is still drawn in and compressed by the piston, expanded as the piston travels downward (acting as an energy-neutral air spring), and then is expelled. This is called Active Fuel Management (GM), Multi-Displacement System or MDS (Chrysler), Active Cylinder Control or ACC (Mercedes-Benz), and Variable Cylinder Management or VCM (Honda). It buys you 1 to 2 mpg.

4. Buy a no-throttle butterfly vehicle. The throttle plate regulates airflow into the engine (think of it as the valve in a faucet), and whenever it's not fully open, it causes fuel-robbing drag. By regulating the timing and opening of the engine valves, and precisely metering fuel flow into the cylinders, it's possible to do away with the throttle plate, thus increasing efficiency. Diesels don't have throttle plates. Neither do BMW gasoline engines.

5. Buy more gears. The more gears in your transmission, the better the economy. Bleeding-edge vehicles are at seven gears and heading for eight; going from four to five gains you more efficiency than from seven to eight, as Lexus is doing. But manufacturing costs, gearbox size, and gearbox weight are all going down, and the higher the fuel costs, the greater your dollar savings when that final gear adds 1 mpg to the economy rating on the window sticker. Continuously variable transmissions are also good. Watch for double-clutch transmissions popularized by Audi and VW to gain wider acceptance. They're automatically shifted manual gearboxes.

6. Buy a TPMS car. Tires underinflated by just 5 psi will drop a 20-mpg car to 19.5 mpg. Woefully underinflated tires, say 15 psi when they should be 30, mess up emergency handling (many 1990s Ford Broncos that flipped had underinflated tires) and wear out faster. Lots of people can't tell 15 from 30—this was director Roman Polanski's problem. Look for a new car with a tire-pressure-monitoring system (TPMS). For your existing car, invest $3 in a cheap tire gauge, $15 in a cool digital tire gauge, or $20 in tire-pressure valve caps that show a different color when you're underinflated by 4 pounds.

7. Buy a minivan. Sexy they're not. Efficient they are. A minivan holds more people, more cargo, has better third room room, and burns less fuel than an SUV weighing hundreds of pounds more. Start by looking at the Honda Odyssey and Toyota Sienna. Ordering them in black or dark grey helps cut the apparent bulk. Somewhat.

8. No child driven alone. One soccer team. Twenty kids. Here's a typical parking-lot scenario for the visiting team: Six 12-mpg SUVs to carry one kid each 10 miles to the game, six other vehicles with one kid each in back, three vehicles with two kids each, one with four kids. A Nissan Armada with four kids in back is better for the environment than a hybrid with one kid aboard. Try carpooling. (Or if practice is half a mile away, have the kids—gasp—walk.) An added advantage of carpools: Kids who decline to discuss their day at dinner get into a carpool vehicle, forget who's driving, and blab about everything.

9. Replace the air filter. A dirty air filter costs you about 2 mpg (10 percent) in fuel economy. At $3 a gallon, you'd get back the price of a $25 air filter in 1,500 miles. While you're at it, have you changed your air conditioner/furnace air filters?

10. Lose the roof rack. Past 50 mph, the engine burns more hydrocarbons pushing aside the wind than dealing with road friction and engine friction. Roof racks cost you fuel and also take 5 to 10 mph off your top speed, which is why police cars with their huge flasher bars sometimes can't catch high-speed speeders.

For more on the prissy sort of check-tire-pressure, drive-responsibly folderol, see the EPA Web site . You could also drive slower, but that gets into deeper issues such as how much is your time worth—a 4-hour trip reduced by half an hour has some value—and also your overall carbon footprint. Al Gore's "inconvenient truth" message about global warming might have gone over better were he not the owner of a 10,000-square-foot house.