"In the history of the world, no one has ever washed a rented car." Think about it: Could there be a pithier way of making the point that if you don't own something and have no stake in its long-term future, you're not going to take care of it the same way you would if it was yours? It's suitable both for New York Times columns by Tom Friedman (he's quoted it many times, usually attributing it to Harvard's Larry Summers) and for toastmasters the world around (you can find it on the "Stories for Speakers and Writers" blog, where it's sourced to former Microsoft executive John Wood, quoting Harvard Business School Professor Michael Porter).

I got to thinking about it the other day when I got back from the carwash with my rented car.

Alas, I am quite confident from my subsequent research that I will not go down in history as the first person to wash a rental. In fact, there was quite the parade of confessions to rental-car washing, contra this very maxim, on Berkeley economics professor Brad DeLong's blog a few years back. One person was motivated to do so by the £10 fee one company charges if you don't return the car clean. Another wanted to get rid of the evidence that he'd been using the vehicle off-road in violation of the rental agreement.

But the more interesting cases, and I would immodestly include my own, are of those moved to wash a rental not to avoid a penalty but just because it got dirty. Sorry, all you professors, columnists, and toastmasters, but in most cases, indeed the vast majority of cases, a better explanation for why people don't wash rental cars is that the cars don't get very dirty.

After all, rental car companies always wash the cars before they send them out. They don't always achieve showroom standards of detailing, but in general, with regard to cleanliness, the experience of getting into your rental car is about the same as getting into your own car after it comes out of the carwash--if not better (here, I'm thinking of those permanent stains the children left on the backseat).

Now, how long will you be keeping this car? Well, if you are on a business trip and using your American Express corporate card to pay for it, according to a 2005 AMEX report, you had it on average 2.9 days. Avis has reported an average rental length of about 3 and a half days.

I don't know how often you have been moved, three or four days after going to the carwash, to take your own car back to the carwash. But it strikes me that this would be extravagant even for Tony Shalhoub's "Monk" character on USA Network.

Mark Thorsby is executive director of the Chicago-based International Carwash Association. His market research surveys show that, on average, people say they go to the carwash twice in a 30-day period. Two-week vacations are not the norm for car rentals. And besides, Thorsby says, "People think they go to the carwash more frequently than they do." He notes somewhat ruefully that if people really came twice a month, they'd be "lined up around the block 24 hours a day" at his members' outlets.