Putting the truck vs cars issue aside, there is still an incentive to make large wide trucks. The fuel efficiency allowance is based on the vehicle's "footprint

Putting the truck vs cars issue aside, there is still an incentive to make large wide trucks. The fuel efficiency allowance is based on the vehicle's "footprint" which is the distance between the wheels side to side times the distance between the wheels front to back. This metric comes out to some number of square feet. The fuel efficiency allowance is proportional to the number of square feet. Larger trucks are allowed to consume more fuel without penalty. Now it turns out that making a truck, say, longer by 10% does not increase the fuel consumption by 10%. If you are a manufacturer, you want to maximize the ratio of the allowance based on the square footage divided by the actual measured fuel consumption of the vehicle. The sweet spot comes out on the large side. So the fuel efficiency regulation, ironically, is causing a trend that leads to more total fuel consumption.