You're ignoring the difference between "raising fuel efficiency standards" and actually achieving, under the immutable laws of thermodynamics and economics, such an increase.
California mandated that 3% of vehicles sold in the state be pure electric vehicles by about this year or so. Where did that get them? Absolutely nowhere, and billions of dollars were poured down a dead-end rathole in pursuit of a practical electric vehicle because of their shortsighted rules. Mandates are merely ways for politicians to pretend they're doing something, as opposed to having any relationship to the real world.
Uh, because all fuel usage isn't done by cars, genius.
What is illogical about it is that it is not fact. We would not cut our consumption of oil in have by cars getting two times better mileage. Why? Because automobiles run off of gas or diesel for the most part. Out of a 42 gallon barrel of oil, only 23 gallons of gasoline is refined. Diesel/fuel oil is also a product. But many other chemicals come out of that barrel.
Secondly, automobiles are not consuming all the gas or diesel. Trains, trucking, power plants all run off of diesel. Airplanes basically run off of kerosene, a derivative of crude oil.
You want to make a dent in oil comsumption, do something significant. Convert all our electrical demands to being supplied by Nuclear power. Wind energy where it is viable. And solar if it is practical.
That would be a good first step.
Only if you can get away with the logic assumption of "everything else remaining constant", which is not true here. If you cut the cost per mile of travel, people will travel more miles.