Car Speeds and CO2

Speeds of cars greatly affect their fuel consumption and therefore the amount of CO2e they release per mile. Slight changes in speed can raise or lower miles per gallon and CO2e per mile by 20%. The slogan, 52 saves CO2, reflects the most efficient speed for cars. Manufacturers optimize cars for 46-58 mph, since most of the EPA Highway test and 6% of the City test are at these speeds.

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.

Car magazines and manufacturers need to provide similar data on MPG at different speeds for new cars. Boating magazines and builders regularly report MPG by speed for boats (fuel.BoatWakes.org), so the lack of data for cars is surprising. In your own car, a trip computer would find its best speed, and encourage you to drive frugally.

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.

Interesting chart of CO2 from transport and fuels.