We’ve all done this: You drop someone off in front of a store so they can run and grab something quickly, so you sit there in the car and let it run idle. If the errand is less than 10 seconds, that’s fine, but any more than that, and you waste gas. That’s according to the Environmental Defense Fund, which says motorists who sit idle for more than 10 seconds waste more fuel and money than turning the engine off and on. Park your car and turn off the engine next time. And don't be tempted to "warm up" the car in the cold winter months -- you waste too much fuel.