Last August, the Obama administration announced it would increase fuel-economy standards to 54.5 mpg for cars and light-duty trucks by 2025. But six long-held fuel-economy myths prevent better efficiency with current automotive technology.

Only small cars are economical: Not so. Newer, standard-size vehicles have advanced technologies such as hybrid drive trains, diesel engines, turbochargers and low-rolling resistance tires, allowing them to compete on the fuel-efficiency charts.