Last week I set a personal best for mpg on a trip we made to and from Philadelphia. Our car records the mileage continuously from the time the tank is filled until it is filled again. On the trip back, we stopped at the Chesapeake House rest area on I-95 at Mile Marker 98 in North East MD. From there to our house in Arlington we traveled about 94 miles. The mileage reading shown when we parked in front of our house is shown to the left.
Over that distance we averaged 50.9 miles per gallon, which is the best mileage I've ever gotten (by a lot) over a similar distance.
A couple of factors worked in our favor. Probably the most important was the weather. It was cool enough outside to not require any air conditioning, which generally reduces mileage by 10-15%. Also, traffic was fairly smooth and not congested, which allowed me to travel in ways that maximized efficiency. In particular, I used the downhills to gain speed and allowed the car to slow somewhat on the uphills. We generally drove about 60-65 mph while on I-95 and the BW Parkway..
Driver behavior is a huge factor in fuel efficiency, accounting for 20% or more of the difference in fuel economy from person to person. I have seen this demonstrated between my wife and me. Our mileage on average differs by about 10%, a difference I can only attribute to our driving behaviors.
Since our car provides constant and instantaneous feedback (it shows real-time fuel efficiency on a gauge next to the speedometer), it makes it easy to train yourself to drive more efficiently over time. This type of feature included in all cars could help improve the overall fuel economy of the US fleet by several percent--conceivably enough to reduce our oil imports by 1 million barrels per day.
So crunching the numbers, it required approximately 1.85 gallons of gas to cover that 94 miles, costing about $7 at current prices. For comparison, a car that averages 35 mpg would have cost $10, while a car that averages 25 mpg would have cost about $14 for that same portion of the trip.