A study of cars popular in the 1990s showed their most efficient speeds were usually 46 - 53 miles per hour, with one car less and one car higher. We have not found more recent data, but the pattern has probably not changed much, since the EPA test has not changed.
Speed (MPH) with Best Fuel Efficiency
Fuel Efficiency (MPG) Achieved at This Speed
31 40 Subaru Legacy
46 45 Geo Prizm
46 28 Chevrolet Pickup
51 33 Mercury Villager Van
53 35 Oldsmobile 88
63 25 Oldsmobile Cutlass
A graph and supporting data show MPG at speeds from 0 to 75 mph, for each car in the study above. The data come from West, McGill, Hodgson, Sluder & Smith, "Development and Verification of Light-Duty Modal Emissions and Fuel Consumption Values for Traffic Models," Oak Ridge National Laboratory March 1999.
Driving 52 mph also reduces stress on the car, which hits bumps less hard, and on the driver. For each hour which could be driven at 60 mph, driving 52 mph adds 9 minutes. For each hour at 70 mph, driving 52 adds 21 minutes.
EPA tests: The EPA highway test for cars is primarily at 46-58 mph, plus one start and one stop, so the average speed is 48 mph. The testing laboratory adjusts resistance on the wheels to reflect wind resistance and weight. EPA reports 78% of the lab mpg to adjust for hills, etc., which are not measured in the lab.
The EPA city test has frequent acceleration & deceleration between 0 and 30 mph. 6% of the test time is cruising at 55 mph. EPA reports 90% of the lab mpg to adjust for hills, potholes, etc., not measured in the lab.