There are more than a few pieces of controversy buried in the Obama administration’s 600-plus-page proposed rule (excluding supporting documents) dictating a uniform U.S. standard for fuel economy and for tailpipe emissions of greenhouse gases for model years 2012 through 2016.

There is not likely to be much argument about the numbers for 2016: the average for all passenger cars, SUVs, and light trucks must be at least 250 g/mi of CO2 equivalent emissions, and the corporate average fuel economy (CAFE) standard is set at 35.5 mpg. But the proposed rule will be contested by both auto manufacturers and environmentalists on whether the new standards will make cars less safe, whether CO2 emissions for electric vehicles are properly accounted for, whether the tailpipe tests are accurate, and many other issues.

The proposed rule from the U.S. EPA and NHTSA (National Highway Traffic Safety Administration) puts flesh on the bones of a conceptual agreement reached last May between those agencies, the state of California, and automakers. A uniform national GHG/CAFE standard would avert the need for automakers to sell separate fleets for California (and states adopting the California GHG standards), with the so-called “California cars” requiring higher CAFE numbers than the federal government was supporting: 35 mpg by 2020. The Alliance of Automobile Manufacturers agreed to accept a higher CAFE standard sooner, in 2016, in return for one national GHG/CAFE standard.

Charlie Territo, spokesman for the Alliance, says the 35.5 mpg average for all autos sold in 2016 was exactly what the Alliance expected. That number could turn out, however, to be as low as 34.1 mpg if the automakers take advantage of certain credits that will be available for improvements in air-conditioning systems and for selling flex-fuel vehicles. What the Alliance did not agree to last May were the interim CAFE averages for years 2012-2015. Territo declined to say whether the Alliance is happy with the interim-year numbers. “We have 60 days to review those figures,” he said. “The document is 1000 pages long.”

But one industry official in Washington says the industry’s ability to actually meet those 2016 numbers will depend on 1) new fuel-efficiency technologies working as expected, and 2) consumers swallowing the costs of those technologies, with the knowledge that they may recoup those costs via resultant fuel savings over a period of years.

The federal government believes that all the technology needed to get today’s autos to 2016 GHG/CAFE levels already exists. But Jim Kliesch, a Senior Engineer at the Union of Concerned Scientists Clean Vehicles Program, said the problem is that auto manufacturers “are not packaging these technologies together.” So they might use cylinder deactivation in one model, continuously variable transmission in another, stoichiometric direct fuel injection in another. Kliesch points approvingly to Ford EcoBoost technology, a combination of turbocharging and direct fuel injection, which the company is phasing in to 90% of its models by 2013. The V6 EcoBoost provides up to a 20% increase in fuel economy with no loss in engine performance, according to Ford.

The EPA and DOT assessed the cost and benefits of applying 35 technologies to various “footprints” and came up with a GHG/CAFE target for each one. Footprint is determined by multiplying the vehicle’s wheelbase by the vehicle’s average track width. For example, the Honda Fit has a 40-ft2 footprint. The 2016 targets for the Fit and any other models (from Honda or any other automaker) with that footprint are 214 g/mi of CO2 equivalent and 41.4 mpg. The 53-ft2 Chrysler 300 would have targets of 270 and 32.8 mpg. These are “targets”; the actual requirements for each footprint will depend on the number of units produced within each footprint.

This footprint-based assessment, pushed by NHTSA, has the potential to result in manufacturers producing lighter cars, which could have a negative effect on auto safety. The proposed rule states: “…there is still risk that manufacturers will rely on downweighting to improve their fuel economy (for a given vehicle at a given footprint target) in ways that may reduce safety.” For example, an automaker with two models in a given footprint might be inclined to stop production of the heavier (and theoretically safer one in terms of mass) to meet that footprint’s target.

Beyond concerns about safety, GHG and CAFE testing methods are also an issue. To measure fuel consumption, NHTSA uses a dynamometer to measure the amount of CO2 and other carbon compounds emitted from the tailpipe; it does not directly measure the amount of fuel consumed during a vehicle test. It is difficult to directly measure how much fuel is consumed by an auto for a number of reasons—for example, because it is very difficult to precisely measure the small amount of fuel that is consumed over a 7.5-mi dynamometer test run.

The carbon exhaust measurement method, carried out in a dilution tunnel, is “very precise,” said John German, Senior Fellow and Program Director, International Council for Clean Transportation and a former Honda America executive. The carbon content of the test fuel is then used to calculate the amount of fuel that had to be consumed per mile to produce that amount of CO2. In addition to the shortcomings of a 7.5-mi test distance, the dynamometer test also suffers some external shortcomings, related to the conditions under which the test is run, including the fact there is no hard acceleration at high speeds; air-conditioning is not considered; and there is no testing of the vehicle in cold weather.

Leaving out A/C systems is a disincentive when it comes to improving CAFE performance, since manufacturers don’t get credit either for substituting greener refrigerants or, more importantly, given the impact on fuel use, for such mechanical features as cycling the compressor on and off, which most A/C systems are not capable of doing, according to German.

These shortcomings explain why the two agencies state in the proposed rule: “Both EPA and NHTSA are interested in developing programs that employ test procedures that are more representative of real-world driving conditions, to the extent authorized under their respective statutes. This is an important issue, and the agencies intend to address it in the context of a future rulemaking to address standards for model year 2017 and thereafter.”

While the emphasis in the proposed rule is on better performance of conventional gasoline vehicle engines, its treatment of plug-in hybrid and electric vehicles has sparked a controversy, too. The proposed rule, for example, does not count heat-trapping emissions associated with generating electricity—from coal plants, for example—to charge those vehicles. For the purposes of calculating GHG emissions for a given manufacturer’s fleet, the EPA would assign a value of zero emissions to every plug-in and electric vehicle. Kliesch said those types of vehicles should be held accountable for their share of such emissions.

NHTSA and EPA jointly will hold three public hearings on the proposed rule: Oct. 21 in Detroit; Oct. 23 in New York; and Oct. 27 in Los Angeles.