The technique that seems to return the highest MPG in my GT auto, in most simple terms, is to keep your foot off the brake pedal and off the throttle pedal as much as possible, while keeping the engine RPM below 2,000. Momentum is the key to efficiency. When you hit the brake pedal to stop quickly, you are destroying the car's momentum. When you accelerate quickly, you are fighting the car's momentum.
* Accelerate with the typical speed of traffic, keeping the engine RPM below 2,000.
* Anticipate stops by taking your foot off the throttle pedal earlier than necessary so that you can take advantage of the car's momentum and engine braking / automatic fuel cutoff. At 55 MPH it is easy to coast a 1/4 mile to a stop sign without upsetting the driver behind you. You might want to tap the brake pedal a couple of times to let the cars behind know that you are slowing down.
* Use the cruise control when possible.
* Get the car into 6th gear as soon as possible. The GT auto will hold 6th gear at about 38 MPH, spinning the engine at about 1,000 RPM.
* Find a quiet road once or twice (or a couple more) a tank and blow the carbon out of the engine by running the engine RPM up to redline though a couple of gears, and then coast back to the speed limit.
My Mustang might be a freak, but the above seems to work. In 3 years of ownership my car burned 1,133.278 gallons of fuel traveling 30,638.4 miles for a combined city/highway average of 27.03123 MPG. 1.5 years ago the car had 7 back to back 400+ mile tanks on a 7 day cruise (99.9% off freeways and interstates), averaging 31.387 MPG. I had one tank of fuel that lasted 466.3 miles, returning 32.29 MPG on another cruise, but that was atypical.
If you want to test the pulse and glide method, use the above technique for a quarter tank to achieve close to 27 MPG on the dash. Then, from a stop sign use the pulse-glide method to bring the car to 55 MPH. You are almost certain to see the car's MPG readout on the dash drop by 0.5 to 1 MPG (or more) with that one experiment. If I want poor MPG, I will just borrow the 2004 F-150 that I gave to a family member - that truck returned 13.7 MPG on average for the last year that I owned it (the front driver's side brake was dragging).