We've all heard of those crazy drivers who will tack on an extra 20 miles to their commute in order to save a penny per gallon of gas. Do you think they even realize that they're guzzling their savings away with the extra mileage?

But I recently came across a way to save a few pennies on gas the requires no extra driving. How? By putting away the plastic and pulling out the cold, hard cash.

My husband had already inserted our credit card and started pumping the gas at a Shell station on the outskirts of Detroit when I noticed an offer I'd never before seen. Pay by credit card and gas was $2.91 a gallon. Use cash and it was six cents cheaper.

It was too late for us that time, but I'll be keeping my eye out for this in the future. Has anyone else noticed this offer before?