Go figure.

That's all we can say to reports that U.S. fleetwide fuel economy in June fell for the third straight month, despite the fact that hybrid and alt-fuel vehicle sales are about double what they were a year earlier.

Fleetwide fuel efficiency last month fell to an even 23 miles per gallon from 23.2 in May and from 23.3 mpg in April, TrueCar reported. Those numbers are consistent with the University of Michigan's Transportation Research Institute's (UMTRI) figures, which show June's Corporate Average Fuel Economy also dropping for the third straight month to 29 miles per gallon.

TrueCar blames what it calls an "unseasonable drop" in gas prices, which apparently caused consumers to buy more trucks and other big-engined vehicles. Average gas prices in the U.S. fell about 15 cents during the month to about $3.35 a gallon as of July 1, according to AAA.

The good news is that fuel-efficiency numbers are still ahead of year-earlier figures. TrueCar has average fuel economy up 6.5 percent from a year earlier, while UMTRI reports a 3.9 percent rise in fuel economy.

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