On our first trip to Yosemite National Park in the Prius, the average miles per gallon worked out to 48.0. But lately, commuting and puttering around town has been getting 46 and change. The only change car-wise is the addition of a bike hitch, which adds maybe 30-40 pounds total. That doesn't seem like enough to account for the difference.
I read that underinflated tires can lower MPG by about 2% for each pound of lost pressure. The Prius recommended pressure isn't written on the wheels, and it isn't listed on the door jamb or in the owner's manual. It's actually on a sticker on the inside of the glove compartment. Turns out the front and rear tires should be 35 and 33 PSI respectively, and they were about 2 pounds low according to the tire gauge.
Normally, a figure like "2% better gas mileage" is hard to get my head around. But with the Prius getting close to 50 MPG, that 2% is around 1 mile per gallon. Sure enough, inflating the tires an extra 2 pounds resulted in an extra 2 MPG for this trip, clocking in at exactly 50.0. (On arrival at Yosemite, we were actually up to 52.)