Governments can make all the laws they want, but they can't repeal the laws of physics. It takes a lot of energy to move cars down the road, you can reduce that some, but you can't entirely get around it. The first battle you must fight to make a car more efficent is with rolling resistance. The biggest factor dictating rolling resistance is weight. American market cars are tubby because we require that they prove their prowness as battering rams before they can be sold. Yup, before a car can be sold in America it has to pass what they call a crash test, a vegistal leftover from the days when folks wouldn't wear seat belts and we tried to protect them with masses of metal. Thanks to our crash tests, even small cars sold in America tend to weight around 3000 pounds, while small cars sold in other markets are much lighter. So we've lost a bunch of the battle of energy efficiency right at the first crash test.
Of course, you can build an aluminum or carbon fiber car, racers do it all the time. But aluminum costs around $2/pound, so an aluminum car would cost thousands more. Carbon fiber? The costs would make new small aircraft look like a bargain, and the layup isn't well suited to mass production. So we can gain a couple percent efficiency by good design, but much of that low hanging energy saving fruit has already been picked. We can also attack rolling resistance with more efficient radial tires, but they're already standard on every new car. If we want to compromise safety we could fit skinny little tires with zero toe in like Toyota did with the Prius, but for most drivers that's an unacceptable compromise.
Next up in the list of energy wasters to be fought is air resistance. Automotive history is full of jelly bean like cars that cheated the winds, but few examples have survived- probably because we have boxy things to haul around in our cars besides ourselves that don't fit well in jellybean shapes. One of the newer tricks to cheat the wind is actual streamlined panels on the underside of a car. These help the new car meet the CAFE requirements, but tend to get busted off on potholes and snowdrifts. There are several new sedans available with aerodynamic co-efficents around .30, a standard that used to be reached by only the most streamlined racing cars. Again, most of the low hanging energy saving fruit here has already been picked.
Last in our trilogy of energy saving strategies is powertrain efficiencies. Back in the bad old days, car cooling fans ran even in the middle of winter and engines struggled to operate efficiently with only three or four gear ratios available. These days it's hard to find automatics with less than four speeds and manuals with five, and six speeds are common even on economy models. Fuel is metered a lot more efficiently too- Precise electronic fuel injection has replaced carbs that relied on wear prone mechanical orifices and venturies to crudely mix fuel and air. Again, the low hanging fruit has already been picked and any major advances will be very expensive... Priced a hybrid battery, diesel injection pump, or turbocharger lately? They each cost in the thousands of dollars.
In practice, getting to the 35 MPG CAFE standard for 2016 is doable- for example the standard 2012 Chevy Cruze @31MPG and Ford Focus @ 30-31MPG are almost there. But just getting the Cruze to 35 MPG requires a $2000 package of an expensive turbocharger and other tricks, and the Focus gets to 33MPG with tricks like closing shutters over the grille to improve aerodynamics for the EPA test. If you live in a hot climate those shutters will probably be open all the time anyway, and up here in the tundra the first snowstorm will probably demolish them.
If you've got an extra $10,000 or so to spend on the initial purchase and thousands more to spend on increased maintainence costs, there are cars out there now that beat the 35 MPG standard by a bit. The Prius and other small hybrid cars better 40 MPG, but before you've made up for the higher initial purchase price it'll be time for a new $5000 battery and maybe a $5000 motor and $5000 controller too. American market diesel cars face the same dilemma, thanks to our toughest in the world diesel emissions standards. The upcharge for a VW diesel engine used to be less than $1000, an investment that quickly paid off with 20 to 30% better fuel mileage. Today with all the complicated pollution control equipment required a new VW diesel will set you back around $5000 more than the equivilent gas engine model.
So given that the auto industry is currently offering $30,000 small cars that get close to the proposed 56 MPG standard, what will it take to get there? Well, VW has already built a smaller than the Golf/Jetta car called the Polo that handily beats the 56MPG standard... But it won't pass our battering ram standard. And I have no doubt that Toyota can build an aluminum Prius that couldn't straddle a speed bump with a turbocharged engine that meets the 56 MPG standard... For $50,000 2011 dollars!
So yes, provided the public, of which not much of us can afford new cars anymore, will buy $50,000 small cars the automakers can meet the 56 MPG proposed standard.
But how about those of us that need to haul tools, farm supplies, and building materials? Now the traditional solution to that dilemma was a 15 MPG full size pickup or van. Well, with a lot of tricks the full size pickups are just edging 20 MPG, and the 2016 CAFE standard is 28.6 MPG. There's not a single pickup or van in the American market that can meet that standard, not even the MPG stars like the diesel Mercedes Sprinter vans or 4 cylinder Ranger pickups that are about to be discontinued. The administration has relented a bit and is demanding only a 3.5% per model year improvement in MPG for trucks between 2016 and 2025. It's possible that a compact pickup with a diesel or hybrid powertrain might be able to meet that 28.6 MPG 2016 standard. But it'd cost $40,000 and couldn't haul a full one ton skid of feed or seed or even a Bobcat on a trailer. And a 2025 pickup? Maybe a Prius with the back lopped off and a 300 pound payload rating would meet the standard...
This all, again, assumes anyone is going to buy these very expensive vehicles with limited capabilities. Granted, there's a small urban market with disposable income that can spend $50,000 on a new car or truck despite the compromises made to meet the proposed 2025 CAFE standards. But some of us have big families and need minivans that seat seven, and some of us need to carry a ton or more of tools and supplies in our work trucks. Granted, some of us have used workarounds like trailers to haul stuff that won't fit in our small cars. But neither the Prius, Focus, or Cruze are approved for trailer towing. in fact, Ford doesn't allow trailer towing with any of their cars; You have to buy a crossover, truck, or SUV to tow a trailer with a new Ford. None of those vehicles will meet the proposed 2025 CAFE standards, and if the standards become law, none will be available by 2025.
So what will be the consequences of the proposed 56 MPG 2025 CAFE standard? A devastated auto industry for a start, as most all consumers won't be able to afford $50,000 new cars no matter how fantasticly efficient they are. Then there's the high maintainence costs of the technologies required to get to 56 MPG- the cost of replacing hybrid batteries, turbos, and diesel emmisions systems will insure that the 56 MPG cars have an early date with the scrapyard. Given that it frequently takes tens of thousands of miles for the latest high MPG car to make up for the energy used in it's manufacture, 56 MPG cars that end up in the scrapyard before their odometers turn 100,000 miles aren't going to save much energy. Meanwhile, we of the downwardly mobile formerly middle class will be forced to drive our polluting gas hogs into the ground.
So while a 56 MPG CAFE standard sounds neat and progressive, in reality it would be a disaster. So let"s tell our president and congress that we want to see steady progress in improving the efficency of our vehicles and switching them to renewable fuels, but we don't want to lose millions of good jobs to do it.