This is a message going around in emails, as it appears on snopes.com (the email scam/myth buster). Who knows if this is true or not, but you can judge for yourself, WITH the "privileged information" at snopes. Anyhow, it seems it wouldn't hurt to do the math at the pump. Whether it's malicious, or just old equipment, the remedy of double checking your tab seems just ordinary prudence.

http://www.snopes.com/politics/gasoline/cheating.asp

But you might want to send THIS instead of that original email that's going around right now. At least if you give them the snopes link, they can judge for themselves whether it's true or not. Snopes hasn't got anything to gain by lying about it.