The EPA estimates that city mpg figures will decrease 12 percent on average, but could drop as much as 30 percent for some vehicles. Highway ratings will also drop about 8 percent on average and up to 25 percent. That’s substantial. The agency expects that gasoline/electric hybrids will suffer the greatest decreases because, due to their often smaller engines, batteries and electronics, they’re most sensitive to cold weather and air conditioning use. For example, explains Millett, drag from a bike rack or the power draw from air conditioning “has a substantial effect on a 1.5-liter engine, but only a nominal effect on a 4.0L.” Even so, if the Toyota Prius’ city mpg rating falls from 60 to 45, that still ain’t bad.