Every month, the University of Michigan sends out its weighted average of the gas mileage of all new cars sold the previous month.

And for January, the news is good.

The new vehicles sold in the U.S. during January scored a sales-weighted average fuel efficiency of 24.5 mpg.

That's up 0.4 mpg from the average in December, and the highest average ever calculated since the index began in October 2007.

It also represents a 22-percent boost from the level at which the index began, five years and four months ago; that October 2007 weighted average was 20.1 mpg.

The university also calculates what it calls its Eco-Driving Index, which looks at the monthly emissions of greenhouse gases produced by an average individual U.S. driver.

This not only looks at the average fuel efficiency, but factors in changes in the average number of miles driven per capita.

That index is now at a level of 0.82, meaning greenhouse-gas emissions per driver have fallen 18 percent since October 2007, which was set as 1.0 on the Index.

The improvements aren't surprising, as the 2013 model year is the second one in which new and more stringent fuel-efficiency requirements are in effect.

The final rules, for model years 2012-2016, were announced jointly by the EPA and the NHTSA in April 2010.

Soon after President Barack Obama took office in 2009, he brought together the auto industry, the various Federal regulatory agencies, and California, urging them to agree on a set of standards for a model year that was less than three years away.

Sales-weighted graph of average gas mileage of new cars sold [University of Michigan]

Sales-weighted graph of average gas mileage of new cars sold [University of Michigan]

A further and even more stringent set of gas-mileage rules takes effect for model years 2017 through 2025, although there's a legislated assessment step shortly after that new round takes effect.

During that step, the industry and regulators will look at the effects of changes to date on fuel efficiency, emissions, vehicle safety, and other factors.

Fuel efficiency information was available for fully 99.8 percent of vehicles sold, according to the university, and model years were assumed to run from September of the previous year through August of the calendar year.


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