Top 10 Fuel Efficiency Myths From MSN Autos
1. A tank nearly full prevents evaporation
While not letting your car run low on fuel may be a good idea if you're traveling in an isolated area, the thought that a fuel tank more full than empty will prevent fuel from evaporating inside the tank is off-base. Cars of yesteryear may have allowed gas vapors to escape into the air, but fuel systems in modern vehicles are designed with vapor recovery systems. Some vehicles with pressurized fuel systems even display a check engine light if the gas cap is loose, missing or not properly sealed
2. Shifting into neutral at stops
This is a myth that may have applied when engines required carburetors. Today's vehicles have computerized fuel-injection systems that sense if an engine is revving above idle when you ease off the accelerator. If that happens, the fuel injectors shut off, so gas is no longer injected into the engine, even if the car is still in gear. The only thing you accomplish by constantly shifting in and out of neutral is premature wear on shift components.
3. Manual transmissions get better mileage
Years ago, your transmission choices were usually a 5-speed manual or a 3-speed automatic. Manual transmissions almost always achieved better mileage because a reasonably skilled driver could control engine revs through efficient shifting. But today's high-tech automatic transmissions are typically more competent and efficient than a driver using the combination of a clutch and stick shift. Few Americans learn how to properly drive a vehicle equipped with a manual transmission, and given that a "stick" is usually reserved for high-performance cars, it's difficult for the average driver to squeeze the best possible mileage out of a car with a clutch.
4. A dirty air filter leads to lower mileage
The engines in older vehicles pulled air straight through the air filter into the carburetor, so a clogged filter could affect gas mileage to some degree. But today's advanced engines have a computerized engine control module (ECM) to precisely regulate the air-to-fuel ratio. In a modern vehicle, air goes through the filter and then through a mass airflow sensor that lets the ECM gauge the airflow and adjust the fuel accordingly; less airflow means less fuel is sent to the engine. While gas mileage may not be affected, dirty air filters can lead to sluggish acceleration.
5. Filling up when it’s cooler gets you more gas
For years, a myth has persisted that if you buy gas in the cooler part of the day — say in the morning during summer — you get more for your money, since a cooler liquid is denser. This theory may sound plausible when you're at the pump during the heat of the day. At filling stations, however, gasoline is almost always pumped from storage tanks underground that are naturally insulated from large temperature swings. Because of this, any slight change in the temperature of the gas is so small you wouldn't notice any appreciable savings.
6. Cruise control saves gas
Since a vehicle's cruise control system is designed to maintain a constant speed, most drivers assume it will help them save fuel. On long highway trips on generally flat terrain, that may be true. But you've probably experienced the sudden acceleration that cruise control systems create when confronted with an incline in elevation. That rapid acceleration burns a lot of fuel, as if you mashed the gas pedal yourself. An alert driver anticipating an upcoming incline will typically apply pressure to the gas pedal slowly as needed to maintain speed without the sudden engine revs caused by cruise control.
7. Fuel additives can increase mileage
Consumers in general and car owners in particular always want to find some magic fix in a bottle. That's why a typical auto-parts store usually has an entire shelf dedicated to various potions promising better mileage. While fuel-injector cleaners may lead to better fuel economy if the ones in your car are dirty and clogged, other fuel additives — even if they work as advertised — probably would not save enough to cover their added cost. To help protect consumers, the Federal Trade Commission has a website dedicated to the doubtful claims of "gas saving" products.
8. Gas mileage drops as vehicles age
If you don't maintain your vehicle, of course its performance will degrade and so will fuel efficiency. But if you keep your vehicle properly maintained and see to any needed repairs, you shouldn't see any noticeable decline in fuel economy. Regardless, some of maintenance items to address as a vehicle gets older include dirty fuel injectors, defective oxygen sensors, worn spark plugs and plug wires, and a defective or leaky gas cap.
9. Topping off the tank helps gas mileage
Do you continue to add gas to your car even after the gas-station pump automatically shuts off, indicating that the tank is full? Many people think that by topping off their tanks they're getting as much fuel as possible into the car and thus can go just a bit farther between fill-ups. The reality is that after your tank is full and your gas nozzle shuts off, any additional gas is drawn into a gas station's vapor recovery system — and back into its storage tanks. And according to AAA, you could even damage your car's evaporative emissions system by topping off your tank.
10. Lowering a truck’s tailgate improves mileage
Drivers of pickup trucks have long assumed that lowering the tailgate is better for aerodynamics and therefore improves gas mileage. But according to Diane Bloch, an aerodynamic-performance engineer for General Motors, driving with the tailgate up is actually more aerodynamically efficient. She says that as air flows over the truck, it falls over the cab and pushes forward on the rear of the truck. The benefits of that airflow are diminished when the tailgate is down. Bloch says that replacing the tailgate with an aftermarket net is worse than having no tailgate at all; she compares it to a boat dragging a fishing net through water.