Reduce the amount of food you discard. Turn down the thermostat by three degrees for eight hours a day. Consume less meat.

Those small tweaks in daily habits will help the United States collectively reduce emissions as the country seeks to stave off climate change as part of an international agreement reached in Paris last year. But of all the things an ordinary person can do to help reduce harmful emissions, the biggest gains can be made in their cars.

A new University of Michigan study puts the onus of meeting that agreement squarely on the shoulder of car buyers and motorists. Researchers at the university's Transportation Research Institute analyzed the impact of dozens of actions individuals could take to reduce emissions and concluded "there is only one realistic action that, by itself, would accomplish the goal." Driving a more fuel-efficient vehicle.

Currently, the average on-road fuel economy of light-duty vehicles in the United States average 21.4 miles per gallon. If the average fuel economy were 31.0 miles per gallon, total US emissions would be reduced by 5 percent, say researchers Michael Sivak and Brandon Schoettle. If the average fuel economy were 56.0 miles per gallon – two MPGs above target of the 2025 CAFE standards – total emissions would be reduced by 10 percent.

That impact would far outweigh some of the more incremental gains from changing eating and consumption habits.

"Improving vehicle fuel economy is by far the most effective action that an individual can take." - Michael Sivak and Brandon Schoettle

Fuel economy in new cars, of course, has declined over the past year. Falling gas prices have given Americans a fresh appetite for gas-guzzling SUVs and pickup trucks. Even if you've bought one of these vehicles, Sivak and Schoette say there are still emissions reductions that can occur. They say each of the following could help reduce emissions by 0.2 percent each: using tires with rolling resistance that is better than current average resistance, reducing the frequency of aggressive driving moves by 25 percent and reducing the frequency of "very high speed driving" by 25 percent.

While different vehicles have their own speeds for achieving optimal fuel economy the UMTRI report says consumption is lowest at about 61 miles per hour. At 87 miles per hour, consumption increases approximately 34 percent. Given the range and frequency of motorists reaching such speeds, the researchers say the average driver can reduce overall fuel consumption by 5 percent by eliminating "very high-speed driving."

In December, the United States joined 195 countries in signing a global climate deal that sets a long-term goal of keeping global warming "well below" 2 degrees Celsius above pre-industrial levels and curbing peak emissions as soon as possible.

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Cars contribute far more than airplanes

Transportation accounts for roughly 27 percent of all US emissions. Within that category, light-duty vehicles – including cars, SUVs, pickup trucks, vans and crossovers – comprise 60 percent of transportation emissions. Medium and heavy-duty trucks account for 23 percent of the transportation emissions. By comparison, aircraft account for 8 percent and rail only 2 percent of the transportation-related emissions, according to the report.

In aviation, the primary factors which influence emissions are the amount of jet fuel spent per passenger-mile, the fuel efficiency of the aircraft and the number of passengers carried per flight. On new aircraft scheduled for use in the "very near future," according to the researchers, fuel efficiency improvements between 15 and 30 percent are expected. Not that the flying public has much of a say in which aircraft flies a given route, but if a person flies on a plane that consumes 10 percent less fuel per passenger mile, that could reduce transportation emissions by 0.8 percent and overall US emissions by 0.2 percent, the researchers write.

A step in a more environmentally friendly direction? Sure. But a small step compared to the gains to be found in automobiles.

"Improving vehicle fuel economy is by far the most effective action that an individual can take, and it would require several other actions to equal the effect of improved vehicle fuel economy," Sivak and Schoettle write.

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