10 Factors That Can Affect Your Vehicle’s MPG


Does your vehicle seem to have a lower fuel economy than it is reported to have?  You may be surprised by how many factors can affect the MPG of your vehicle. 

  1. Some fuels contain less energy than others.  Oxygenated fuels, reformulated gasoline (RFG), and gasoline with ethanol in it can all decrease MPG.  Also, your vehicle will probably have a lower MPG during the winter because typical summer conventional gasoline contains about 1.7% more energy than typical winter conventional gasoline. 
  2. Small variations in the way vehicles are manufactured and assembled can cause MPG variations among vehicles of the same make and model.
  3. The older your vehicle is, the lower the MPG is compared to when you first bought it.
  4. When you first purchase your car, your vehicle will not have reached its optimal fuel economy until the engine is broken in, which may take three to five thousand miles.  Break-in miles decrease MPG five miles.
  5. A poorly maintained vehicle will have a lower MPG.  Make sure the engine is tuned, the tires are properly aligned and inflated, the air filter is clean, and there is no brake drag.  Check out our previous blog post series How to Save $ at the Pump for more information. 
  6. All vehicles are tested in 2-wheel drive.  So if you have a 4-wheel drive vehicle, engaging all four wheels makes the engine work harder, therefore reducing fuel economy. 
  7. The EPA tests all vehicles on flat ground, so if you are driving on hilly or mountainous terrain or on unpaved roads, you will notice a decrease in fuel economy. 
  8. If your vehicle is carrying more than 300 pounds of passengers and cargo, which is the amount all vehicles are tested with, you will notice a lower MPG than advertised.  Also, adding cargo to the top of your car increases aerodynamic drag, therefore decreasing fuel economy. 
  9. Temperatures below 40 degrees Fahrenheit can lower MPG by five miles.  This is because your engine doesn’t operate efficiently until it is warmed up, and it takes longer for your engine to warm in cold weather.  However, you should know that letting your car idle to warm up doesn’t help your fuel economy.  Similarly, frequent short trips can reduce fuel economy because your vehicle operates a smaller percentage of time at the desired temperature. 
  10. How you drive is a huge factor when it comes to your vehicle’s fuel economy.  Though the EPA city test includes idling, excessive idling will lower MPG.  Quick acceleration, heavy braking, driving more than 80 mph, and using the air conditioner or other electrical accessories are also all factors that may affect your vehicle’s MPG. 

It is also worth noting that the reported MPG of a vehicle itself may not be completely accurate.  Testing used by the EPA were last revised in 2007 and are probably in need of an update, especially since it has become more difficult to predict fuel economy for emerging technologies like hybrids, electric cars, and natural gas vehicles.  Check out our previous blog post for more information on the fuel economy of alternative fuel vehicles, which is reported as MPGe.  

4 Responses to “10 Factors That Can Affect Your Vehicle’s MPG”
  1. kay mctavish says:

    my husband drives a 2012 F250 Super Duty 4wd, diesel, he tows a 22 ft commercial flatbed trailor for equipment Pick up and delivery. Does it affect his mileage when he tows the trailer empty?

  2. Eric says:

    I drive a 2006 Toyota Corolla 4 speed automatic. My brother and I just tuned it up. When i tested it recently, it was getting about 25 mpg, but by the advertised mpg it should be getting around 32. I drive it a lot in medium-heavy traffic in the DC area, sometimes 75mph on the highway and i have to spend on average 6 minutes looking for a parking spot at college/at night. additionally, i have been filling it up at Shell gas.

    Would all these factors explain the lower mpg or do you think it might have a bad mass air flow sensor/there is something possibly clogged up in the fuel system?

    Thank you in advance!

    • Hi Eric! That’s a great question. The manufacturer’s advertised MPG is always tested in perfect conditions, with no wind or other weather factors, no traffic, etc. Many things can cause that advertised MPG to drop (as we referenced in our blog post), including using the air conditioner. The brand of gas shouldn’t have a big effect.
      A problem mass air flow sensor would trigger a check engine light, so it probably isn’t that. You might want to check your air filter – that could be a culprit.
      If that’s clear, then it’s probably just a result of your driving patterns. You will get better mileage on highways than in city driving, and any time you spend in traffic will have an effect as well.

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