Highway gas mileage estimates for about a third of all new vehicles sold in the U.S. will fall by one mile per gallon because the government has changed the way it calculates the window sticker numbers.
The change begins with the 2017 model year. It's largely because people are loading up cars with more options such as heated seats that use electricity and create drag on engines. The Environmental Protection Agency also says drivers run air conditioning more often than in the past, and they're driving faster, both of which cut into mileage.
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Initially the change could make it hard for buyers to compare mileage from one model year to the next. But the EPA says it will update numbers on its website in the next few weeks.