How to deal with "You drive, I'll pay for gas"
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.
The hard way is to confront the problem directly. Set better ground rules before agreeing to drive. Assemble good stats (such as provided by Edmunds) and lay out for your friends the total cost you will incur. They will either agree to split the cost equitably, or another of them will agree to drive under the gas-only custom (you win), or the group will reconsider other options such as flying or taking a bus. If they baulk at paying their fair share, perhaps they weren't really friends after all.
The easier way to approach the problem is to rent a car. Let the rental car company accrue depreciation, and split the whole cost of the rental four ways. People intuitively understand this as fair. This tactic has an added benefit; the rental company will be responsible for breakdowns. Renting could also get you into a nicer vehicle than the one you own.
Perhaps you don't mind giving your friends $360. But at the very least, they should understand that you're doing so. Perhaps they'll even return the favor.