@UMTRI shows fuel-economy gains 1970-2010 mask less impressive result accounting for #people/vehicle. Excerpt:
Table 1 presents basic facts concerning personal transportation in 1970 and 2010
based on the information in RITA (2013). During the 40-year period examined, vehicle distance travelled increased by 155% (from 1.674 to 4.261 trillion km). However, because vehicle load decreased by 27% (from 1.9 to 1.38 occupants), occupant distance travelled increased by only 84% (from 3.182 to 5.867 trillion km). Vehicle fuel economy (of the entire fleet of light-duty vehicles) improved by 40% 
(from 18.1 to 10.9 liters per 100 km). However, because of the decrease in vehicle load, occupant fuel economy improved by only 17% (from 9.5 to 7.9 liters per 100 km).
As a consequence of the changes in vehicle fuel economy, vehicle distance travelled, and vehicle load, the total amount of fuel used increased by 53% (from 303 to 463 billion liters).

@UMTRI shows fuel-economy gains 1970-2010 mask less impressive result accounting for #people/vehicle. Excerpt:

Table 1 presents basic facts concerning personal transportation in 1970 and 2010

based on the information in RITA (2013). During the 40-year period examined, vehicle distance travelled increased by 155% (from 1.674 to 4.261 trillion km). However, because vehicle load decreased by 27% (from 1.9 to 1.38 occupants), occupant distance travelled increased by only 84% (from 3.182 to 5.867 trillion km). Vehicle fuel economy (of the entire fleet of light-duty vehicles) improved by 40% 

(from 18.1 to 10.9 liters per 100 km). However, because of the decrease in vehicle load, occupant fuel economy improved by only 17% (from 9.5 to 7.9 liters per 100 km).

As a consequence of the changes in vehicle fuel economy, vehicle distance travelled, and vehicle load, the total amount of fuel used increased by 53% (from 303 to 463 billion liters).