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I've been tracking the mileage on my 2015 VW Golf (a non-intercooled turbo, but not diesel, thank goodness). Of course, my driving habits and highway/city balance influence things greatly, but beyond that the data seems to show significant and durable changes in the mileage I'm getting.

So, I'm wondering: what other factors might influence my mileage in the long term? Some possibilities:

Let's assume my car has no malfunctions. I'd also be interested in which factors might have the greatest effect.

  • Tire pressure is one that's commonly mentioned - under-inflated tires roll less efficiently. – Nate Eldredge Dec 10 '15 at 0:31
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    In at least some areas of the US, the fuel blend is changed between summer and winter. The winter one is more volatile, I think. – George Dec 10 '15 at 2:34
  • @George "More volatile" sounds right (or "less volatile" in the warmer summers), but what would that do to mileage? – Daniel Griscom Dec 10 '15 at 2:50
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Several things come to mind (though I'm sure there's a lot more):

  • O2 Sensor: As the front O2 (or lambda) sensor gets older, it gets lazy (doesn't react as quickly). Due to this, as they approach end of life, you'll start seeing declining gas mileage numbers.
  • MAF Sensor: Your Mass Air Flow (MAF) sensor detects the amount of air being taken into your engine. This should be gently cleaned about every other oil change with electronics or MAF cleaner.
  • Tire Pressure: Low air pressure.
  • Spark Plugs: Most vehicles have a 100k mile replacement on plugs. Once you get past this, you'll start seeing mileage decreases.
  • Winter Blend Fuel: In the US, some States require different fuel blends for different times of the year. The winter blend fuels are notorious for lowering your fuel mileage.
  • Ethanol E85: E85 has about 30% less energy per gallon of fuel. Due to this you need more of it for the same output. If you are running it, you'll see a definite reduction in mileage.
  • Ethanol E10: In the US you can still find stations selling non-ethanol blends. While you won't see the reduction you'll see with E85, you still have less energy per gallon of fuel with E10, so you'll see a hit.
  • Oil Weight: You put down the type of oil (conventional vs synthetic). While this has some validity, the weight of the oil has a lot going on with it as well. A lot of newer vehicles use 0W-20 or 5W-20 oil, which is a lot easier to push through the orifices than 5W-30 or 10W30. While the lesser weight oil will give better gas mileage use the weight your manufacturer specifies for your vehicle. These newer vehicles are designed to handle the thinner oils. These oils will not give your engine the protection it needs if it's not designed to use them.

These are just some I can think of off the top of my head. I'm sure there are a plethora of them which could be cited.

  • clean answer, very nice – DucatiKiller Dec 10 '15 at 0:58
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Some obvious factors that haven't been mentioned.

  • Weight - if you carry a full load of passengers (or other heavy stuff), mileage will drop.
  • Aerodynamics - open windows, add-ons like badly-adjusted spoilers, and dented or missing parts of the body will drag you down, especially when driving fast.
  • AC - adds directly to fuel usage the more often and intensely it is run.
  • Does using the heater (on a gas-powered car) use gas? – Daniel Griscom Dec 10 '15 at 11:46
  • Physics view: The car is closed system. So the heat energy has to come from somewhere. The source of energy for the car is gas. So yes the heat comes indirectly from the gas. But as it is waste heat from cooling the engine I'm not sure how much will heating affect the mileage. – jnovacho Dec 10 '15 at 11:50
  • @DanielGriscom: you're right, I hadn't considered that heating probably uses exclusively waste heat. AC is a different story though. – Michael Borgwardt Dec 10 '15 at 12:16
  • FYI, my Golf has a "Convenience Load" display which purports to show mileage lost to secondary loads. The only thing it shows is A/C, which in my experience shows as a decrease of 0.5 MPG. – Daniel Griscom Dec 10 '15 at 13:01
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Do not forget how long has elapsed since the last vehicle service.

This is important because components like spark plugs require regular replacement to keep your engine efficient.

Also, things like the condition of the brakes can have a massive effect on gas mileage. For example, a brake that isn't releasing cleanly and hangs on slightly is akin to driving with your handbrake engaged.

Other things like your car being clean and polished can have a negligible effect as a dirty car won't slip through the air as easily.

I would suggest, if you are interested, do some further research on "Hypermiling Tips" and you will find whole communities dedicated the achieving the best possible MPG from a vehicle.

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