Calculating energy bill’s real figures:

Gas mileage would go up under the compromise reached by Congressional leaders last week, but not as high as the trumpeted numbers. And despite the tougher 35 m.p.g. standard, a growing population of drivers would push up total fuel use, as well as greenhouse gas emissions — but not as rapidly as would occur without the legislation.

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Even if the bill becomes law, the fuel-economy improvement that it calls for will probably not be great enough to prevent some increase in American fuel consumption because of the expected growth in the number of cars on the road and miles traveled.

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What that requirement would mean for car buyers is not completely clear. A much larger number of vehicles are likely to rely on hybrid power systems combining an internal combustion engine and an electric motor, while more fuel-efficient diesel engines would also proliferate. Plug-in hybrids, relying more on power from the electricity grid, are likely to be on the rise.

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Among the unknowns, of course, is the price of the gasoline that will be saved. If the price is high, either because of market forces or increased taxes, government standards may only have a modest role in promoting smaller or higher-efficiency vehicles, because consumers will already be seeking out fuel efficiency and automakers will have a strong incentive to meet that demand on their own.