An acquaintance of mine recently drove a group of his friends to a convention a couple of states away, probably 1,000 miles round-trip, under the typical 'I drive, you pay for gas' arrangement. It wasn't my place to explain to him that he'd been ripped off by his friends, big time.
Run the numbers. The vehicle they took got around 25 mpg, which means that the trip burned 40 gallons of amber gold. At $4 a gallon, the three passengers split a $160 gas bill, which figured out to $53 and change each. Edumund.com's handy "True Cost To Own" calculator for the Honda CR-V they drove shows that, based on 15,000 miles a year, over five years the cost per mile to operate this vehicle is $.52. This includes depreciation, insurance and maintenance, in addition to fuel.
At that estimate, the trip cost my friend $520. Minus the $160 in gas, he was out of pocket for $360, all for the privilege of chauffeuring his friends.
How can you deal with this situation? The easy way, or the hard way.