Thanks to the rules as written, those numbers on a new car indicating gas mileage are the result of never actually driving on a road.
I had wondered about this, just how the car makers and marketers arrived at their numbers. The law allows for vehicles to be placed on a treadmill - which means once the vehicle actually hits the roads, and traffic, and stop lights -- well, in a typical scenario, the actual mileage was usually half of what the sticker said.
Again, its the rule set in the 1970s by the government that allows for no actual driving conditions to be tested to obtain mileage. The information about real conditions comes from tests conducted by Consumer Reports.
"For example, Chrysler says the four-wheel drive diesel version of the Jeep Liberty gets 22 mpg in the city. Consumer Reports tested it and found it got more like 11 mpg.
Honda claims its hybrid Civic sedan gets 48 mpg in the city. Consumer Reports found it only gets 26 mpg -- a 46 percent difference.
Chevy's Trailblazer EXT four-wheel drive is supposed to get 15 mpg in the city. For Consumer Reports, it was 9 mpg."
Read more here.