U.S. Fuel Economy Is at All-Time High, Researchers Say University of Michigan researchers said Monday that new cars and light trucks sold in the United States in October had the highest average fuel economy ever recorded on American vehicles - 24.1 mpg combined. Michael Sivak, one of the researchers, said in a telephone interview that new vehicle sales showed a four mile-per-gallon gain from October 2007 to October 2012, an improvement of about 20 percent. Mr. Sivak and his colleague at Michigan's Transportation Research Institute, Brandon Schoettle, have tracked data from 1923 to show a largely stagnant fuel economy trend from that time until about 1973, when the Arab oil embargo caused gasoline shortages. That data was published last July in the journal Transport Policy. "Technology improvements were used to add power and acceleration for 50 years, and it's only after that time that we see sharp increases in fuel economy," Mr. Sivak said. The average on-road fuel economy of all vehicles in 1923 was 14 m.p.g., the report said, compared with 17.4 m.p.g. as recently as 2008. Historical fuel economy figures were attained by dividing the amount of fuel used for road transportation by vehicle miles traveled. More recent mileage information comes from Environmental Protection Agency data as used on window stickers. "This is great news for consumers and the environment," Luke Tonachel, a senior analyst in the transportation program at the Natural Resources Defense Council, said in an e-mail. New cars being sold today make up the greenest car fleet ever, and we can look forward to even cleaner cars as fuel-efficiency standards continue to ramp up. If new cars had the efficiency of 2007 models, drivers would be spending another $8 billion a year at the pump. Fuel economy varies widely across the vehicle fleet. According to the Transport Policy article, the average 2011 model year car has 38 percent better fuel economy (23.7 m.p.g. combined) than a pickup truck from the same year (17.2 m.p.g. combined). The best car, it said, is nine times more fuel efficient than the worst car, and the best pickup twice as fuel efficient as the worst. The article also showed that driving a car uses about twice as many B.T.U.'s of energy per occupant mile as taking an Amtrak train on the same trip. Flying also reduces energy use compared with driving, but not by as much. The comparisons take into account the average number of passengers in a car and the average load in planes and trains. Mr. Sivak and Mr. Schoettle also publish a national Eco-Driving Index that estimates average monthly greenhouse gas emissions per American driver, based on fuel economy and per-capita distance driven. The monthly index was unchanged in August, reflecting the most current data available, but has declined 19 percent over all since October 2007.