13 Causes of Poor Gas Mileage (Increase Your MPG)

If your car is suddenly getting terrible gas mileage and leaving you wondering “why is my car using so much gas all of a sudden?”. You’re not alone and today’s gas prices make it a bigger issue than it once may have been.

There are a number of common causes behind plummeting fuel efficiency. Fortunately, many of which can be easily remedied so you can get back those missing miles per gallon.

Causes of High Fuel Consumption

1) Excessive Idling

An often overlooked MPG killer (especially during summer and winter) is excessive idling. While it may seem harmless to sit in your parked car for extended stretches while waiting for someone or letting the engine warm up, you are getting literally 0 miles per gallon during that idling time.

Those minutes idling here and there really add up. It’s been said that idling using anywhere from 0.17 to 0.50 gallons of fuel per hour, depending on the vehicle.

Contrary to the myths, idling for longer than 10 seconds uses more fuel to keep the engine running than it would take to restart it. Unless you are stopped in traffic, it’s best to turn the engine off if sitting more than 10 seconds.

2) Carrying Excess Weight

One simple cause of high fuel consumption is when a vehicle is carrying too much weight, whether it has a lot of items in the trunk, full of people, carrying a roof cargo box, or pulling a trailer.

The more weight that’s being carried around by the vehicle, the more demand that is being put on the engine to generate enough power to move all of it. In the case of a roof rack, you will also have additional wind drag to deal with which also affects your MPG.

Whenever the need for more power is needed, whether cruising or accelerating, the engine needs to burn more fuel to make it happen. Therefore, you end up having to fill up your fuel tank more often.

Keep in mind that excess weight resulting in lower gas mileage is often proportional to the amount of power a car produces. For example, an extra 100 pounds in a full size truck won’t be a big deal but that extra weight will have a big impact on your Smart car’s MPG.

3) Dragging Brakes

If brake pads, calipers, or other component don’t fully disengage from the rotor after you’ve released the brake pedal, you’ll have constant friction and drag as the component lightly rubs against the spinning rotor. This consistent friction forces your engine to work harder to maintain speeds and causes a noticeable reduction in gas mileage.

Often, you’ll notice a burning smell while driving or excessive brake dust covering your wheels. Causes may include stuck calipers, warped rotors, incorrect brake pads, or air in the brake lines.

4) Low Tire Pressure

The tires on your vehicle need to be inflated to the proper amount of air pressure, usually between 32 PSI and 36 PSI on the majority of vehicles out there. A good tire pressure gauge is something everyone should own.

If you’re driving a vehicle with low tire pressure on any or all the tires, it creates more wear and resistance for those tires. This causes the engine to work harder to make up for the additional resistance, which means more fuel will need to be used to power the engine.

5) Worn Tires (Low Tread Depth)

It may come as a surprise but tires with insufficient tread can cause increased fuel consumption. Because they have less grip and traction on the road, it makes the engine work harder when accelerating since some of the wheel motion is wasted.

In addition, tires that are unevenly worn (inner or outer tire wear) adds extra drag and reduced mileage. This is because patchy tire wear alters the contact patch with the pavement, creating imbalance.

6) Dirty Air Filter

There are many reasons to have a clean air filter, but one big reason is to have good fuel economy. If your air filter is clogged or dirty, less air makes its way into the combustion chamber making the engine work a lot harder to satisfy the power requirements of the driver.

This may even cause a running rich condition where the air/fuel mixture is not optimal. So, remember to change your air filter at the recommended interval set by the vehicle manufacturer. Your vehicle manual will have this information.

7) Driving Too Fast

Believe it or not, the difference between going 80 MPH and 70 MPH could mean the consumption of 25% more fuel. The two biggest factors that contribute to this are wind resistance and gear ratio. That is why it is always best to just drive the speed limit or be as close to it as possible. Otherwise, you will be using up more fuel than needed.

8) Aggressive Driving

Even more important than driving above the speed limit, aggressive acceleration and braking have a drastic effect on your MPG.

This style of impatient, impulse-based driving can reduce MPG by a staggering 33% compared to gradual, steady acceleration and braking practices. Slamming the pedal for bursts of speed taps much more deeply into the less efficient ranges of your engine’s RPMs and fuel consumption.

Intense acceleration also often necessitates sudden braking to lower speed for corners or when closing in on traffic. This starts a vicious cycle reminiscent of frustrating stop-and-go commute driving, constantly see-sawing between flooring it and then braking hard.

Smooth acceleration will save you a lot of fuel, especially when there’s another red light a couple blocks ahead.

9) Improper Gear Changes

This is especially true with manual transmissions but also applies to automatics with sport modes. Don’t change gears too early or too late. If you try to upshift at too low of an RPM, the engine has to work harder to accelerate.

Alternatively, if you constantly let your engine run in the upper RPM range in your powerband since you enjoy the extra power, keep in mind that you’re also burning a lot more fuel.

10) Old Engine Oil

Not only do your spark plugs need to be in good shape, but you must ensure that you regularly change the oil and replace the oil filter as well. Over time, engine oil becomes thicker and has different flow properties.

Because of this, there is increased resistance within components of your engine. This leads to more fuel being needed to move a vehicle.

See Also: How to Dispose of Old Gasoline

11) Air Conditioning

When you use the air conditioner, more demand is put on the engine. As a result, it must burn more fuel to meet this demand. Use standard ventilation from the blower if possible or simply roll down your windows when driving at slower speeds like on typical city streets.

Keep in mind that driving with your windows open also burns more fuel because the openings create a dragging effect which slows the vehicle down. This is especially true at higher speeds such as on the highway. In that case, using the AC is the better option.

12) Bad Oxygen Sensor

Your engine has oxygen sensors which keep track of the mixture of air and fuel in the internal combustion chamber. Based on what it senses, it tells the powertrain control module to add the correct amount of fuel.

But if the oxygen sensor is faulty, the system may automatically add more fuel even if the engine doesn’t need it.

13) Bad Spark Plugs

When spark plugs get worn out, you will have more engine misfires with your vehicle which will use up more fuel. You’ll want to use good quality spark plugs possible (OEM plugs are often best), so you can get the most miles out of them. Iridium spark plugs and platinum spark plugs are the two most preferred types.

How Weather and Environmental Factors Influence Gas Mileage

Cold Temperatures

• Frigid winter air makes engine oil viscous like molasses. This creates more internal friction to overcome.
• More gas is burned getting the engine up to operating temperature.
• Winter gasoline blends have higher ethanol content which lowers the fuel’s energy density per gallon.

Hot Temperatures

• Warm summer heat helps engines avoid cold start issues, but running the A/C full-time places a heavy accessory load on the engine – reducing highway mileage by up to 25%.

High Winds

• Intense headwinds or storm gusts produce significant aerodynamic drag on the vehicle exterior, making it much tougher to maintain speed.
• The engine has to burn extra fuel to battle those wind resistance forces trying to push it back.

Rain and Snow

• Wet roads cause tires to hydroplane more easily, increasing rolling resistance.
• Hard throttle stabs or wheel spin accelerate wear and greatly reduce mpg.

Altitude Changes

• Traveling into mountainous elevation alters air density and oxygen content.
• Thinner air causes engines to lose power and efficiency at altitude.
• More throttle is required just to maintain speeds, hurting fuel economy.

Monitoring fuel consumption across different seasons and weather illustrates how dramatically external conditions can impact MPG. What first appears to be a random gas mileage plunge often traces back to environmental factors.

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Kevin

Kevin has been fascinated by anything automotive since his first go-kart as a kid to his time working as an auto tech for Dodge after obtaining his degree in automotive technology. When he's not working on cars or writing about them, you can find him and his E36 M3 at an event with the local chapter of the BMW CCA.

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