Moreover, the high fuel efficiency of subcompacts is important when calculating a manufacturer's fleet average to determine compliance with Federal fuel standards.

A problem for the Democrats is that, once there is no fleet average, the debate gets murkier because there is no single number to argue over.

Japanese manufacturers achieve higher fleet averages primarily because they sell smaller cars.

Q. What do you think is an attainable automotive fleet average?

Under the law, each automaker must maintain minimum fleet averages for its cars and trucks to avoid penalties.

Theory and Reality In theory, the fleet average would lower pollution by as much as a requirement that all cars meet a maximum pollution standard.

The existing fleet averages over 364,000 life miles per vehicle.

But the current fleet averages 8 to 9 years, and some are 15 years old.

The current fleet averages about 4.5 mpg fuel efficiency.

One idea under consideration, government officials and car company executives say, would be to do away with the existing system of fleet averages and set a standard for each vehicle.