I had heard about the concept of hyper-mileage for years, but felt that it had no place in my commute, since the tactics that I'd heard most about were the more radical ones, like over-inflating your tires and driving without air-conditioning. (this is not an option for me for the majority of the year since I live in the United States Sunbelt)

In 2010, I made a combination-business-and-pleasure trip to California and Nevada and got a screaming deal on a compact car rental. (don't let them pressure you to upgrade)  The car was a 2009 Chevrolet Cobalt and to my initial displeasure came without cruise control.  It did have another feature that proved to be highly instructional over the next twelve days that I drove the car; an instantaneous fuel mileage calculator.

When I first picked up the car, the cumulative fuel mileage was an alarming 22 MPG, due to the always higher-than-the-national-average fuel prices in California.  The fuel economy indicator could be toggled between cumulative and instantaneous and I found that my instantaneous mileage was much better than the cumulative mileage when I first got the car.

This is when the light went on.  We don't get 30 MPG continuously throughout a tank of fuel; we get 6 MPG when accelerating away from a traffic signal and 99+ MPG when coasting down long grades.  All this averages to the 30 MPG we calculate when we fill up.

Previously, when trying to stretch my fuel, I relied on cruise control (and drove with it most of the time) to keep my velocity at a virtuous 55-58 miles per hour.  Still, I couldn't get more than the EPA rating for my car at the time, a 2004 Honda Accord.

After my experience with the ability to see instantaneous fuel mileage, I realized that although cruise control maintains a constant pace, which would seem to be more efficient than a human driver constantly accelerating and decelerating to stay with the flow of traffic, it does not provide the best fuel economy possible.

The key to getting better mileage is not driving slow or maintaining a continuous speed; it's improving the below-average fuel mileage portions of your driving so that they don't drag the higher fuel mileage portions down as severely.  This is accomplished through management of inertia.

Putting inertia into a car takes lots of fuel if you try to do it too quickly.  The  instantaneous fuel mileage indicator told me that I could get up to 25 MPH using 7 to 9 MPG for about five seconds.  Once at 25 MPH, I back off on the acceleration and still gain 1 MPH per second while getting 24 to 25 MPG.  This has the effect of reducing the 0-60 MPH time on the car from ten seconds to about forty seconds, but increases the fuel mileage average on acceleration to roughly 20 MPG, which is much better than 6.

The second way that inertia can be managed is when climbing inclines.  Even in places with the flattest terrain, like Florida, there is some natural variation of the level of the road and artificial variation caused by things like overpasses.  Because cruise control just maintains constant rate, it uses more energy than necessary to top hills; it only reduces input when past the crest and velocity is too high.  Because of their mass, a car will lose inertia quite slowly when acceleration stops. (despite gravity)  Using this principle, we can improve the below-average fuel economy incurred by rising terrain.

First, establish the range of speed that you want to drive.  You may be surprised that it is possible to get where you are going in a acceptable amount of time and get better fuel mileage, within reason.  Generally, this range will be 10-15 MPH with the top rate you wish to travel as the top number in the range. (you wouldn't want to get a speeding ticket just trying to save fuel)

Although we really want to maintain our pace on rises, this can take fuel mileage down to 12 MPG (instantaneous) in an economy car, depending on steepness of the grade.  If we are willing to lose a little inertia as we approach the top of a hill, we can regain it almost immediately on the other side without using unnecessary fuel.

On level ground, try to maintain your speed in the center of the desired range.  As you approach an incline, about ten seconds before the grade increases, add as much power as you feel you will need to maintain pace up the slope.  This will allow you to take advantage of easier acceleration on flat ground and should let you store inertia in the car by reaching the maximum range speed as you begin the grade.  Slowly remove power as you reach the top of the range and let gravity absorb the motion so your velocity begins to fall as you climb the rise, but maintain the speed in the middle of the range when you reach it.

When you are about ten seconds from reaching the crest of the slope, allow the inertia to wane so you reach minimum range speed as the grade turns down and we can use gravity instead of fuel to restore our pace.

At the bottom of the slope, allow the car to coast until the velocity approaches the center of the desired range gradually increasing the throttle until the center speed is maintained.

Shortly after I picked up the car at the rental office outside Los Angeles International Airport, (LAX) I began to play with the instantaneous fuel mileage calculator, seeing that I could greatly affect the numbers depending on how I drove.  On the first leg of the trip, from LAX to Las Vegas, I was able to achieve 40 MPG (conventionally calculated with the fuel receipt and odometer) in the Cobalt, which has an EPA highway rating of 33, including some city driving on the way out of LA and in Las Vegas.  On the return leg from Las Vegas to Ontario, California, I calculated 44 MPG.

I've since traded in the 2004 Accord and now have a 2010 Honda Civic, which for me is a great commuter car.  It has comfortable leather seats, satellite radio, a USB port for my personal audio, and driven carefully gets great fuel mileage.

P.S. You can compare fuel economy of different models at https://www.fueleconomy.gov