Gasoline not only fuels the vehicle, but it also functions as a coolant for the electric fuel pump motor. In modern cars, this pump sits in the middle of the gas tank filled with cool gasoline. A near-empty tank can cause the fuel pump to suck in air and overheat, causing increased wear to the pump.
Each time you neglect pumping gas, gunk from the bottom of the fuel tank could get caught in various components of the vehicle. This sediment in your tank can foul the fuel filter. If the fuel filter doesn’t catch the sediment, you run the risk of clogging a fuel injector. This is less of an issue today with modern heavy-duty plastic fuel tanks, but in older vehicles with metal fuel tanks, rust particulate can be a serious danger.
When a diesel runs dry, the injector pump fills with air and the vehicle won’t start by simply adding more diesel fuel. A tow truck and mechanic may need to get involved to tow the vehicle, remove filters, pressure-blow the fuel lines before adding fuel and priming the engine. Because of this, maintaining a full tank with a diesel engine is even more critical than with a gasoline engine.
The most common concern with allowing your fuel to run low arrives once the mercury dips below freezing. That’s because the condensation that builds up in your fuel tank when it’s low has a greater chance to get sucked into the fuel lines and freeze. Not only can this mean your vehicle won’t start (with frozen water blocking the flow of fuel to the engine) but the lines themselves may become damaged by the expansion and contraction of the freezing water.
2. Gas Mileage
Some will argue that because the gas tank is lighter when its low, a low tank will give the car more fuel efficiency. A lighter load does require less gasoline, but the weight of a full fuel tank is not significant. A gallon of gasoline weighs only 6.3 lbs. (for reference milk weighs 8.6 lbs. per gallon), so even in most large vehicles the total weight of a full tank is under 250 lbs. Your car may actually be less efficient when the tank is near-empty, as more air in the tank can increase fuel evaporation.
Running out of fuel completely is the biggest danger of allowing your tank to run low, especially during the temperature extremes of winter and summer. Additionally, when an engine dies brakes and power steering can be lost, so running out of fuel at highway speeds can be hazardous in itself. Besides, delaying the expense of a fill-up doesn’t reduce long term expenses, so it is never worth becoming stranded and putting yourself or your family in danger.