Don’t count on reserve fuel if gauge is on ‘empty’

now, and those seem to have less in reserve and are pretty accurate. But cars with traditional fuel gauges have, on average, a gallon or two left in the tank once the gauge reads empty. It’s what the manufacturers call “moron insurance.”

Gauges are designed that way so that if you mess up and forget to get gas, and suddenly notice that the tank is on empty, you’ll still have enough fuel to make it to a nearby gas station. The system was vehemently opposed by the Tow Truck Drivers’ Association, who felt it was severely cutting into their 401(k)s. Carmakers rarely talk about the gasoline reserve, with good reason; they want you to forget about it.

It’s like that emergency hundred dollar bill you stuffed into a hidden pocket in your handbag. If you were always aware of it, it’d probably be a pair of shoes by now. Similarly, it’s better not to count on any reserve fuel and be blessedly relieved that it’s there when you need it. But remember, “a gallon or two” is just our estimate. If you want to know exactly how much fuel remains in your own tank after the gauge reads empty, there’s only one way to find out.

Download the latest “Car Talk” podcast so you have something to listen to, put two gallons of gas in a gas can, and start driving. When the gauge hits “E,” reset the trip odometer to zero.

Then check it again when you coast over to the side of the road. Divide that number by your miles per gallon, and you’ll know how much of a reserve you have.

And then forget all about it, and fill up every time you get down to a quarter of a tank.