Congress recently passed an increase in fuel efficiency standards for cars, from 25 MPG to 35 MPG, a 40% jump. So you would expect that, when this law goes into force, gasoline usage will go down. That’s what various congresspersons and “environmentalists” are arguing, anyway.
Now, the mandated increase is a very large change, and complying with the law is probably beyond current engineering capabilities. That is, automotive engineers will have a difficult time implementing these standards in the time alloted, unless they do the one easy thing available to them, which is to make cars lighter. Lighter cars get higher gas mileages.
Making cars lighter is not hard. You simply take things out of heavy cars or make smaller cars. Problem solved!
Except smaller and lighter cars, all other things being equal, fare far worse in crashes. People know this, and tend to buy a larger vehicle instead. That is, confronted with a choice of a small, more dangerous, car, they will more likely buy a larger SUV or a truck.
Trucks and SUVs do not have to comply with the higher gas mileage requirements. Mileage for these larger vehicles is about 15 MPG (average of city and highway driving).
So instead of buying a safer car that now gets the required 25 MPG, people will be more likely to buy vehicles that are, on average, 60% less efficient!
Thus, more gas will be used than before the higher standards were in place.
Of course, I cannot prove that my scenario is certain to happen, but it is at least not impossible, and even somewhat likely. If I am right, this will be yet another example of good intentions gone bad.