Why did my MPG drop?

A change in your fuel economy could be caused by mechanical problems with your car, but it could also be caused by many other factors. This page attempts to explain some of the reasons your mileage might change, as well as ways to improve your mileage.

Non-mechanical reasons for poor mileage

  • Why would MPG decrease if not due to a mechanical problem? There are several non-mechanical factors that can contribute to poor mileage.
  • Cold weather reduces fuel economy. During winter it can be normal to experience a drop in mileage of 5.3%.
  • The seasonal use of oxygenated fuel can account for another 3% reduction in mileage.
  • Taking short trips will have a large effect on your MPG.
  • A change in driving style can account for 31%
  • A change in driving speed from 65 MPH to 55 MPH can account for a reduction of 14%.
  • A change in commute (hilly vs. flat) can affect fuel economy. (no % numbers found)
  • A change in traffic conditions can account for another 10.6%.
  • The viscosity of lubricating oils will affect mileage. According to one estimate, mileage could be improved be by 5.8% if mechanical friction could be reduced by 50%. No type of oil will reduce friction by 50%, but thinner oils will certainly have some effect.
  • Measuring error due to inconsistent gas nozzle shut off can account for a perceived reduction of up to 3% on a single tank.

If you were to add all of these up, it might account for a mileage reduction of 50% with a perfectly tuned car.

cold weather decreases MPG

Fuel economy is reduced when the ambient air temperature is low. There are several reasons for this:
Fuel atomizes poorly when cold. The fuel must be atomized to be burned in the combustion chamber. If a smaller percentage of the fuel atomizes, more fuel must be added to provide enough atomized fuel to burn. Therefore all cars run a “rich” mixture when they are cold, the colder the engine, the richer the mixture. The engine will eventually warm to operating temperature (about 190 deg. F), but the colder the engine is, the longer it will take. Even after the engine is fully warmed up, cold ambient air will continue to cool as it mixes with the fuel, causing poor atomization.
Lubricating fluids become thick when cold, and add resistance to motion. Think of how hard it is to run in waist deep water. Now imagine waist deep Jello! The thicker motor oil puts a drag on the crankshaft, cam shaft and pistons, and the transmission and differential gears must spin while immersed in the honey-like oil.
Warm up. Some people let their car warm up before starting to drive. The fuel used to warm the engine, but not move the car is fuel wasted. Unless the car won’t drive properly until warmed up, there no reason to spend more than 10 seconds warming a car up.

WInter gas blends

Beginning in October or November, gas companies at an oxygenate to the fuel to decrease tailpipe emissions. The oxygenate additives, unlike hydrocarbons (gasoline), do not store energy, and therefore will can not be burned to provide power in the engine. If 3% of every gallon has no energy, you would also expect a 3% drop in miles per gallon.

Short trips decrease fuel economy

As mentioned earlier, when an engine is cold, the air/ fuel mixture is rich, meaning more fuel is used to do the same work. If a car is driven on a series of short trips, each one starting with a cold engine, the mixture will be rich for most of the miles driven and will use a LOT more fuel than if the car was filled up and driven until it was almost out of gas in one very long trip. The more trips per tank, the more cold engine driving, and the lower the mile per gallon. The fewer trips per tank, the less cold engine driving, the higher the miles per gallon. This trips per tank should always be considered before spending money on mechanical diagnosis.

How you drive has a huge effect on your MPG

Your car’s engine control computer tries to maintain an air fuel ratio of 14.7 parts air to 1 part fuel. When the throttle is opened a small amount, there is a proportionally small amount of fuel added. When the throttle is open wide, there is a proportionally larger amount of fuel added. The less you open the open the throttle, the less fuel you will use.

Obviously, there are times when you must press the gas pedal further down to get the car to go at the necessary speed. However, the way you step on the gas will affect how much fuel you use. When the throttle is suddenly opened, the computer adds and extra surge of gas to prevent the engine from stumbling. The more often you stomp on the gas, the more often the computer will pump extra gas into the engine and the worse your mileage will be. Rapid brake wear and poor mileage usually go hand in hand. People who accelerate more than they need to will need to brake more often. Accelerating gently, only as needed, will make a large difference how far you can go on a tank.

Wind resistance

Remember the “55 Saves Lives” campaign in the 70s? I’ve been told that it had more to do with the fuel shortage than the desire to decrease auto accident deaths. Regardless of whether that’s true, going 55 MPH instead of 65 MPH will increase your miles per gallon by 14%. This, of course, assumes you drive on the freeway all the time, so it’s unlikely that you could expect a 14% increase in fuel economy by driving 55 MPH alone, but if you did half of your driving on the freeway, you could expect a 7% reduction, and that’s nothing to sneeze at. The reason for the drop in mileage at higher speeds is wind resistance. The faster you go, the harder it is to push the air out of the way.

Driving up hills burns more gas

I think most people intuitively understand that it requires more fuel to drive up a hill than it would require to drive the same distance on flat ground. Just as it takes more of your energy to pedal a bike up a hill than to pedal along on flat ground. However, they may not consider their recent change to a hilly commute when thinking about their car’s decrease in mileage.

Stop and go traffic kills fuel economy

If you are stuck in stop and go traffic, your engine continues to burn fuel when it’s running, and since you are not moving, you get zero miles per gallon for every gallon burned while stationary. This is why the hybrids all have the idle stop feature.

Using the wrong weight oil can cost you money at the pump

Viscosity is essentially thickness. Water would have low viscosity relative to honey. Engineers pair bearing clearances with viscosity, the smaller the clearance, the lower the viscosity. It’s important to use the type of oil the manufacturer had in mind when designing the car. However, there is sometimes more than one manufacturer approved viscosity option. When this is the case, using the thinner of the options will increase the mileage a little. Synthetic oil may also have a small effect on mileage (although probably not enough to justify the added expense of the synthetic oil).

Inaccurate calculation of MPG

How to accurately check your miles per gallon

  1. Fill the tank until the nozzle clicks off.
  2. Reset the trip odometer.
  3. Drive until the tank is low.
  4. Fill the tank until the nozzle clicks off.
  5. Note the quantity of fuel added including the 10ths.
  6. Note the trip odometer reading.
  7. Divide the trip odometer mileage by the gallons added. 8) Make a note of the mileage.
  8. Repeat steps 1 through 8 at least 4 times.

Why step 8? When filling the tank, an air bubble will be trapped in the top of the tank that will make the point at which the nozzle shuts off inconsistent even if the same nozzle is used each time the tank is filled. Add to the equation that every nozzle and pump combination is likely to be a little different, and mileage calculated over 1 tank could easily be off by 5%. By averaging mileage over several tanks will reduce the impact of nozzle shut off variation as well as reducing the impact of different types of driving you might be doing (one week, lots of freeway because of a road trip, next week a lot of stop and go because of road construction on the way to work, etc.)


In an ideal world, it would be best to check your mileage when the car is still new (or new to you at least). The best way to do this would be to take mileage measurements under a variety of conditions (short trips, long trips, winter, summer, freeway, around town, in the hills and on the flats). This way you have a range of miles per gallon that can be considered normal. I’ve never done this myself, and I wouldn’t blame you if you don’t have the energy for it either.

What is important is measuring you mileage before bringing the car in. Even if you don’t have a baseline to compare it against, this information is still useful to us, since we have a general idea of what mileage to expect from the makes and models we repair.

It’s also important to think about reasons your mileage might have changed. A different commute? A different primary driver? Cold weather? A new camper shell? New tires? Using the AC more? Be sure to let us know about any factors you think might have affected your mileage.

Of course, we can do a mechanical check for things that might cause poor mileage any time, with no preparation on your part. However, it costs money, anywhere from an hour to two labor hours, depending on how thorough you want us to be. What we’re trying to avoid is spending a bunch of your money to tell you your car is fine.


Up to this point in the article, all of the reasons for bad gas mileage have had nothing to do with the car itself, and that’s appropriate since most of the time the reason in a drop in mileage has nothing to do with the condition of the car. However, there are some mechanical problems that will ruin you gas mileage. Let talk about those now.

Tire pressure

The number one reason for poor mileage is low tire pressure. Anyone who has ridden a bicycle with a low tire knows, as soon you stop pedaling the bike starts to slow. Fill the tires up and the bike seems to coast forever. Cars are the same way; a low tire will put a real drag on a car.

Sometimes it’s hard to know the proper pressure for a tire. The tire pressure listed on your tire information label (usually found in the glovebox or on the driver’s door jamb) may have a pressure that is much too low for the tire that’s been installed on your car. Usually there will be a note about the pressure required for a given load rating in the tire manufacturer’s catalog. Although, there is not always a catalog handy when you need to set your tire pressure.

Here’s our rules of thumb for setting tire pressure: tires with a max pressure (found on the sidewall) of 32 PSI, should be filled to 28-30 PSI, tires with a max pressure of 35 PSI, should be filled to around 32 PSI, tires with a max pressure of 44 PSI should be set to 36 to 38 PSI.

These are rules of thumb, tire wear patterns are the final word. Tires that wear on both outer edges are under inflated Tires that wear only in the center of the tread are over inflated. (Tires that continue to wear on both outer edges despite being filled to near their max pressure fully warm have an inadequate load capacity and should be replaced with tires capable of handling the weight of your car)

The type of tires you buy will affect your gas mileage

As discussed above, low tire pressure will increase a tire’s rolling resistance, but that’s not the only reason one tire might roll easier than another: tire design and size will affect rolling resistance as well. Generally, tires with a higher load rating are more likely to have lower rolling resistance than a tire with a lower load rating assuming they are of equal size, and many tires are marketed as low rolling resistance.

Also, tire size and tread will affect fuel economy. Just as a 10 speed with skinny tires will coast longer than a mountain bike with fat knobbies, your car will roll easier with thinner tire. If you are willing to sacrifice some traction and stability for some fuel savings, you may want to choose the thinnest manufacturer recommended tire size and choose a highway tread.

Sensor drift can cause poor fuel economy

Internal combustion engines run well with rich mixtures. It’s not uncommon to find a car that runs just fine but sucks a lot of gas. In the days before computer control, cars were designed to run a little too rich all the time. Too little gas (a lean mixture) will cause a car to run poorly, whereas the mixture would have to be grossly rich before a car will start to run poorly. Without computer control, there was no way to accurately trim the mixture to 14.7 to 1, so the designers erred on the rich side.

The O2 sensor was one of the first sensors monitored by computer control systems. The O2 sensor sits in the exhaust stream and sends a signal to the computer: over .6 volt if too much gas is being added, under .6 volt if there is not enough gas being added. The computer would use this signal to control a solenoid in the carburetor that would reduce or increase the mixture as necessary.

Cars have come a long way since then, and the last carburetors were phased out by the late 80s. However, the O2 sensor, and its newer incarnation the air/fuel (AF) sensor, are still in use today, and still play an important role in increasing fuel economy and reducing emissions.

When O2 sensors fail, they can’t read rich and are slow to respond to changes. This translates into a rich mixture most of the time, which will cause your car to use more gas. Some of the newer computers will notice when an O2 sensor starts to wear out, but most will just keep on adding too much gas. When a car is getting poor mileage, the O2 sensor should be checked. This is done by checking the fuel trims with a scan tool.

Out of time engines get bad mileage

There are two types of timing: ignition timing and cam timing. Ignition timing refers to when the spark plug sparks in relation to the position of the piston and crankshaft.

Here’s yet another bicycle analogy: the bicycle’s pedal is like the piston, the crank is like the crankshaft, and your foot is like the explosion initiated by the spark plug, pushing down on the piston. If while riding a bike, you step on the pedal too far into the downward stroke, when it’s nearing the bottom of its travel, and you will not get all of the power possible out of the stroke. If you step on the pedal before it crests at the top of the stroke, then you will spin the crank backwards. Correct ignition timing seeks to start the explosion at the time that will put the most downward force on the piston just after it crests and is beginning its downward stroke, just as you would do to the pedal with your foot when you ride a bicycle.

Ignition timing

Because the air/fuel mixture explosion takes a while to develop maximum pressure, the spark must be started before the point at which maximum pressure is desired. The air fuel mixture will take the same amount of time to explode regardless of engine speed. However, the time until the piston reaches the best position for downward pressure decreases as the engine speed increases. Therefore, the spark plug must start the explosion earlier and earlier as the engine speed (RPM) increases. This is known as the “advance curve”. Both the base timing (the crank position at which spark is initiated at idle with no computer control) and the advance curve must match the manufacturer’s specification to have optimum engine efficiency and fuel economy.

Cam timing

Cam timing refers to the time when the intake and exhaust valves open and close in relation to the piston and the crankshaft. If the am timing is off, it will also affect engine efficiency. Newer engines with low resistance timing chains sometimes have issues with chain stretch, especially if the motor oil isn’t replaced on a regular schedule or inappropriate motor oil is used.

Tire alignment can affect fuel economy

Wheel alignment, most notably the alignment angle called “toe”, will increase the car’s rolling resistance. Imagine standing with your feet parallel. If you were to move your big toes closer together, that would be “toe in”. If you were to move your big toes away from each other, that would be “toe out”. Toe in and toe out will both slow the car down by “scrubbing” the tires against the road instead of rolling over it. Toe that is out of specification will cause a distinctive wear pattern on the tires. If there is no wear pattern on the tires, toe probably isn’t the issue.

Dragging brakes

If the rear brake shoes are over-adjusted, the engine will have to fight the brakes to move forward. The same is true for brakes that stick on, which can be caused by a binding caliper or wheel cylinder, or an emergency brake cable that’s binding. Brake drag is easy to test for. Lift the car with all 4 wheels off the ground, mash the brake pedal several times, yank the e-brake several times, then spin all 4 wheels by hand. If they spin freely, there is no drag.

Spark plugs and ignition coils

If the secondary ignition system does a poor job of igniting all of the air/fuel mixture in the combustion chamber, all of the unburned air and fuel are pumped out the exhaust pipe and do not contribute to the power generated by the engine. I believe worn spark plugs are blamed for poor fuel economy more often than they should be. “Poor gas mileage = “needs tune up” in many people’s minds. That being said, secondary ignition can cause poor mileage and should be checked when more common causes have been eliminated.

Motor wear can decrease your gas mileage

As an engine wears, some of the explosive force meant to push down on the piston top will begin to slip past the piston and into the crankcase. The amount of explosive force that slips past the piston is known as blow by. The percentage of the explosion that leaks out as blow by does not contribute to the power the engine generates. And the fuel used to create that portion of the explosion that is blow by is wasted. There is no quick or inexpensive fix for a worn engine. So other more correctable problems that might cause bad fuel economy are usually checked first. The engine’s mechanical condition can be gauged by doing a compression and/or leak-down test. Most manufacturers publish specifications for compression.

Slipping transmission / Lock up converter / Slipping clutch

If some of the rotating force (torque) generated by the engine is lost in a slipping transmission or clutch, the fuel used to generate the force is wasted, and will not translate into movement of the car. People usually notice a slipping clutch or automatic transmission fairly quickly after it begins. However, the lockup torque converter found on most modern cars with automatic transmissions is quite subtle in its operation, and a malfunction may not be noticed from the driver’s seat.

The torque converter is analogous to the clutch on a manual transmission. It allows the engine to continue to spin while the car is in gear during stops. It does this by allowing slippage at lower engine RPMs. While slippage is desirable when waiting at a stoplight, it’s wasteful when cruising on the freeway. The lockup torque converter stops slipping by locking the converter into a solid unit when conditions are acceptable. Locking is usually controlled by the computer via a solenoid. If the computer, solenoid, sensor, or converter malfunctions, poor mileage can result.

Wind-drag due to accessories

I think most people intuitively understand wind resistance. The more forward-facing surface area a car has, the more wind drag it will have. The more drag it has, the more gas your engine will have to burn to counter the drag. Your car is shaped the way it’s shaped. There’s not a lot that can be done to improve its aerodynamic profile without major cost. You can consider the shape of your next car before you buy. Until then, consider the aftermarket add-ons you install. Camper shells and other add-ons that increase wind resistance and vehicle weight.

Decreased hybrid battery capacity

If you have a hybrid, the battery capacity will affect your fuel economy. It’s normal for battery capacity to decrease over time. Just like cell phones and laptops. As battery capacity drops, less regen energy can be stored. Typically, hybrid vehicle fuel economy will decrease by about 20% over the life of the battery. Then, after a new battery, the original fuel MPG is restored.

Mileage improver scams

Every once and a while a customer will bring us a mileage improving device to install. We’ve also found a few already installed on cars in for service.

I’ll make this statement with no qualification. There are no devices that can be easily added to your car that will improve mileage.

How can I be sure? Imagine a device that would improve mileage, even by 1%. It could be licensed to auto manufacturers and the inventor would be rich. If the device worked, it would already be installed on every new car. After the inventor was enormously rich from licensing his technology, he might work on selling retrofit kits to the general public for their older cars.

Why would anyone waste their time selling a device to the public first when they could make a lot more licensing it? Because the device doesn’t work, and the average consumer isn’t equipped to verify the device works. Some of the devices are cheap (Harbor Freight sells one for under $5.00 (which is $5 more than it’s worth). Some devices are very expensive. We had one customer who spent $300 for a piece of clear plastic with a sticker that looked like a circuit board stuck to the bottom.

Don’t buy into these scams, just check your tire pressure regularly and keep your car maintained.