We’re making progress in reducing greenhouse emissions from new cars in the U.S. Is a 14 percent gain since October of 2007 impressive? You bet it is, considering that population continues to grow, and we’re not making significant progress in cutting our miles on the road. Or enacting any climate legislation, for that matter.

The 14 percent reduction is enshrined in the latest monthly Eco-Driving Index from the University of Michigan. Michael Sivak, a research professor at Michigan, told me that this is a “significant” drop in just four years. He said the index measured a two percent decline in per capita driving, which is also significant because vehicle miles traveled went straight up for decades. The index found a 12 percent improvement in new car fuel economy, which is reflective of wholesale abandonment of V-8-powered SUVs and trucks in favor of four-cylinder compacts.

The green hero in all this is, in fact, the oil industry. Why? Because they jacked up prices above $4 a gallon repeatedly during the period studied, and that changed consumer behavior far more than any environmental guilt-trip could. Americans are driving (a little bit) less, and they’re taking those drives in much more fuel-efficient cars. The correlation between fuel efficiency and greenhouse emissions is exact, which is why the Obama Administration is lumping them together.

In the first quarter of this year, the average American car’s fuel economy jumped to 22.7 mpg, up 1.9 percent from where it was at the same time last year. It’s a small jump, but it’s highly significant over the whole auto fleet. Small cars are now 19.4 percent of total vehicle sales in the U.S. The bestselling car in the U.S. is not some big SUV, it’s the compact Chevrolet Cruze (at left). Also selling well are such gas sippers as the VW Jetta and the Hyundai Elantra. The latter achieves 40 mpg on the highway, and people are buying ‘em like hotcakes.

Sivak said that the 14 percent drop in his index swamps the four percent increase in drivers over the period. According to U.S. Census data, the population was 301 million in 2007, and 311 million now. So that’s nine million more Americans in just four years. An actual decrease in emissions from cars as population grows faster than ever is kind of a landmark event.

The caveat over these gains is that they’re incredibly sensitive to changes in fuel prices. And you can’t measure the effect easily. In June, sales of what are called “advanced powertrain” cars fell 30 percent from the same month a year ago—so are slightly lower gas prices responsible? No, the fuel price drop was too small (something like 30 cents a gallon, to $3.55 a gallon nationally) to have that kind of effect. The real culprits were a) Continuing Japanese car supply problems from the earthquake/tsunami, affecting the Nissan Leaf most prominently, and b) The Chevrolet Volt’s month-long production hiatus as it gears up to, you guessed it, actually make more cars.


Christopher Hopson of IHS Automotive told me, “Slight movements in gasoline prices usually don’t move things drastically, quick spikes and certain trigger points ($3 or $4 per gallon) have been shown to change consumer sentiment, neither of which we saw last month.”

Overall, there’s something to celebrate here. Driving is down, and people are drawing greener cars. And they’re changing their behavior in numbers big enough to affect the Eco-Driving Index. It’s good news.

Jim Motavalli ( @jmotavalli ) writes about cars, technology and the environmental world to anyone curious enough to ask.

Cars now getting better climate grades
According to the University of Michigan's Eco-Driving Index, CO2 from new cars in the U.S. is down 14 percent from 2007. That's a significant gain, especially s