"When you hit the brakes, it is creating heat which is being displaced. Once you change your mindset and coast into lights and stop signs, not only is your fuel economy going to go up, but you're actually becoming a more courteous and safe driver," says Gerdes.

Hypermilers tend to drive a bit slower, and in a world where most interstates have a flow of traffic moving along at 80 mph, some argue that going 55 mph could be dangerous. Gerdes says he drives 55 miles per hour on 65-mile-per-hour highways and doesn't see a problem. Sticking to the right hand lanes, he uses a technique called a "ridge ride" where he puts the right wheels of his vehicle against the fog line and rides near the edge of the lane. Gerdes says this position makes the vehicle stand out and drivers coming up from behind can see that he is moving slower.

"If people think that we are holding up traffic or it is impossible to do, it is not. I've driven in L.A., San Francisco, Salt Lake City, New York City, Chicago and Miami all in the past four months and I haven't changed my driving habits," says Gerdes.

Some can be dangerous

Reed says drivers with manual transmission can glide in neutral on some highways, although it may be illegal in some states. Reed once conducted an experiment in his 2007 Honda Fit with "short shifting," a practice where one puts the car into higher gears at a lower rpm. Driving a seven-mile loop and shifting at 3,000 rpm (instead of his normal habit of shifting at 4,000 rpm), he saw a 5.5 percent increase in fuel economy on his ScanGauge. Results can vary by vehicle but, in general, the lower a car's engine speed, or rpm, the less fuel it will use. Short shifting is generally most effective in city driving at lower speeds and stop and go traffic where one might ride in a gear or two higher than in normal driving. The vehicle will have less power but will gain in economy .

"If you have a stick shift, you can pick up a huge amount (of fuel economy) by gliding and short shifting," says Reed.

Other hypermiling techniques may be safe and legal but aren't necessarily comfortable. Many hypermilers avoid air conditioning altogether or go through an almost scientific process of bumping up the fan as they shift into higher speeds. Gerdes recommends turning off the air conditioning while accelerating from a dead stop then turning it on once the vehicle gets moving.