Back in my car rally days, we used to compete in an automotive event where fuel efficiency was the winning criterion. We would fill up at the start and end, drive around at prescribed average speeds for a few hours under a variety of road conditions, and the winner was determined by mileage (handicapped by the weight of the car and other factors from a table put out by the sports car racing clubs).
We used to practice for this event by driving around in traffic with an empty coke bottle (in those antedeluvian times, coke came in glass bottles) standing on the floor boards. If your starts, stops, accelerations or turns were too extreme, it would fall over. When you got to the point you could drive in traffic at reasonable speeds without knocking over the bottle, you were using the minimum fuel possible.
Off Topic » in reply to 5 mpg more.
The old coke bottle trick