Ever wondered how the nation as a whole is doing with respect to fuel economy? The chart above shows the average fuel economy of the national passenger car fleet as a function of time. It is plotted in Gallons Per Hundred Miles (GPHM) versus the year. The raw data is available at the Bureau of Transporatation Statistics.
Remember that GPHM is the amount of gasoline the car uses for every 100 miles it travels. So to estimate the total gasoline used in the nation for a given year by passenger cars, you would have to multiply the number in the chart above by the total passenger car vehicle-miles for that year. The good news is that the chart shows a steady decrease. This is good because it means that each year the typical car uses fewer gallons to go a hundred miles. The bad news is that the curve seems to be flattening out. There was a big improvement in the 80's, dropping from more than six gallons per hundred miles to only five in 1990. But during the 90's up until the present day, it only dropped about another half gallon, to 4.5 gallons per hundred miles. Good that it is dropping, bad that the rate of dropping is getting smaller.