How much fuel could you save just by driving differently? Between 5 and 15 percent, according to some studies, and researchers are testing approaches that they think could boost the total savings to 20 to 25 percent.
So-called eco-driving combines devices that give you instantaneous feedback on your gas mileage as you drive with a suite of tips for driving to squeeze more miles out of a tank of gas.
Meanwhile, in new approaches, researchers are testing devices that offer trip advice about what route to take and what speed to drive to maximize fuel savings.
Studies have shown that when cars are equipped with a device that provides information on instantaneous fuel consumption -- standard in a growing number of cars including the Toyota Prius -- participants typically cut fuel use by anywhere from 5 to 15 percent.
"When people get instantaneous feedback, they can say: 'Oh, look how bad my mileage was," said Matthew Barth of the University of California, Riverside. They can also see the driving changes that bring improvements in their mileage.
The continuous feedback is important. When people just get advice on driving in ways that save fuel -- like avoiding quick starts or slowing down on the highway -- they tend to fall back into less efficient driving habits. "The problem with the static advice is it doesn't really stick," Barth said.
The mileage feedback encourages more gradual starts and stops and slower highway speeds.
With eco-driving, "you become a less aggressive driver," said Jack Barkenbus of Vanderbilt University. "It encourages you to get your foot off the pedal a little bit more. You'll find that on the interstates, going slower really makes a difference."
Barkenbus said that a 10 percent decrease in fuel usage -- and therefore in driving-related CO2 emissions -- is a realistic possibility. If one-third of Americans did this, he estimated in a paper published last year, it would save the equivalent of taking almost 6 million cars of the road, or eliminating seven large coal-fired power plants. Those who dropped their usage by 10 percent would save around $200 to $400 per year.
To push the approach further, Barth is now testing higher-tech approaches to eco-driving. His team is testing a device that uses real-time traffic and road grade information to tell drivers the best route to take to their destination and what speed to maintain during given traffic conditions to save the most fuel.
Barth said that these "eco-routing" approaches could add another 5 to 10 percent improvement in fuel economy, which would bring the total potential for eco-driving closer to 20 to 25 percent savings.
Barkenbus considers eco-driving to be distinct from " hypermiling," a similar approach, but, he noted, one in which some drivers achieve more extreme feats of mileage by doing unsafe things like turning off their engines while coasting down hills or drafting as close as possible to semis on the highway.
"Eco-driving is really consistent with the safety perspective on vehicles," he said. "Insurance companies should love it because if people drove in an eco-driving fashion there would be fewer accidents."
There are currently no federal programs to promote eco-driving, Barkenbus noted in his paper. He suggests driver education about eco-driving strategies combined with subsidies for onboard instantaneous mileage meters could help encourage the approach.
Meanwhile, here are some eco-driving tips from ecodrivingusa.com -- sponsored by the Alliance for Automobile Manufacturers and the Environmental Protection Agency -- for improving your mileage even without a device to help:
-- Plan your trips to minimize distance.
-- Don't let the car idle to warm it up. Modern cars don't need this step.
-- Avoid quick starts and stops. Aggressive driving can lower fuel economy by as much as a third at highway speeds and by 5 percent around town, according to the EPA.
-- When you can do it safely, keep moving. Accelerating from a full stop uses more fuel.
-- Drive your car's optimum speed -- usually around 60 mph -- when you can. Every 5 mph over 55 mph decreases fuel economy by about 7 percent. This is the equivalent of paying almost a quarter more per gallon for every 5 mph above 60 mph, according to EPA.
-- Cool the car appropriately, depending on your speed. Below 40 mph, opening the windows is the best approach. Above 40 mph, the air conditioning is better because the increased drag from having the windows open increases fuel consumption by more than the energy needed to run the air conditioner.
-- Maintain your car: Read the owners manual, tighten the gas cap (to prevent fuel evaporation), use the recommended grade of motor oil or "energy conserving" oil, and keep the tire pressure up. Around 1.2 billion gallons of fuel were wasted by under-inflated tires in 2005, according ecodrivingusa.com.
-- Keep unnecessary weight off of or out of the car. Roof racks and spoilers create drag that reduces fuel economy and excess weight requires more fuel to pull it around. Depending on the car's weight, an extra hundred pounds of cargo decreases fuel economy by up to 2 percent, according to the EPA.