Q: How far can my car run when the gas gauge says empty? I know a light is 
supposed to come on and the manual says the car can go another 40 miles,
but just how much safety margin is built into the system? How many gallons
are left in the tank? — Cindy in Fredricksburg, Va.

A: No two different makes/models have the same tank configurations, nor are
they necessarily calibrated to turn on low-fuel lights when certain amounts
of gas are left within them. However, a rule of thumb is that when the
light goes on (or the gauge says empty) there are two gallons left in the
tank. That doesn’t mean you have two gallons to use, though, because the
fuel pickup tube isn’t flush with the tank’s bottom because it would suck
up water and debris. The best thing to do is believe what the manual says.
If your manual says you have about 40 miles left, you’d better start
looking for a gas station.