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by JEANNE FLEMING, PH.D. and LEONARD SCHWARZ

Question: I'm part of a group of four people who carpool 265 miles round trip to a once-a-month workshop. We take turns driving our own cars - except for one guy, Steve. He says he can't because he travels an extra 100 miles round trip to meet up with us, and that's too much driving. Steve does pay for a quarter of the gas each trip, but I don't think that's enough. One of the other drivers agrees with me; the other doesn't think it's important. Who's right?

Answer: Certainly not Steve. As a car owner himself, he surely understands that paying for gas is only part of the cost of operating a vehicle. Insurance, maintenance, depreciation - these things add up. As a matter of principle, Steve has an obligation to figure out the true cost of that 265-mile journey and kick in his fair share.

Here's an estimate of how badly he's underpaying. Let's say each of your cars gets 20 miles to the gallon and gas costs $3.85 a gallon. Then the gas outlay for the round trip totals about $51. But the Internal Revenue Service, which is fairly reliable on this point, estimates the full cost of operating a car at 51¢ a mile. That makes the actual cost of the round trip about $135, not $51 - and puts Steve's fair share not at the $12.75 he's paying now but at $33.75. So Steve is shorting the owner of the car about $20 a trip, not to mention the occasional thank-you gifts he should be giving each of you for serving as his chauffeur.

A reasonable question, though, is whether 20 bucks is worth making an issue of. Only you and your fellow drivers can answer that. But if we were in the car pool, we'd vote for telling Steve he needs to buy lunch.

Questions? Email Money Magazine’s ethicists – authors of “Isn’t It Their Turn to Pick Up the Check?” (Free Press) – at FlemingandSchwarz@right-thing.net.