Cars will cost more due to the 54.5 mpg regulations. The government says drivers will save more than that on gas. Auto dealers aren’t so sure.

How will the average mileage for car manufacturers meet this rigorous standard? No one really knows. The 54 mpg standard is achieved under lab testing only. Actual mileage will be high 30s to low 40s. Natural gas engines are weighted different than gas. Manufacturers can get credits for improving air conditioning. This sounds a bit convoluted, doesn’t it?

The new regulation isn’t strictly a fuel-consumption rule. Rather, it limits the amount of carbon dioxide a vehicle may emit to 163 grams per mile, Hwang noted. The amount of carbon dioxide coming out the tailpipe, however, is directly related to the amount of fuel burned.

Truckers, whose diesels are increasingly regulated with mandated emissions reduction say the new diesels are already more expensive to mainstain and get worse mileage.