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I've been getting gas for my 2010 Ford Taurus at Get-go and always was able to get a full tank. The front panel indicated 420 miles left to empty

Example: enter image description here

But now for a couple of months, I wasn't able to get over 400 after filling my tank full. Now I get around 370 miles. What is causing this? I was thinking maybe gas is creating a foam and forcing the sensor to detect the level incorrectly. Another reason I thought maybe the quality of the gas got bad and the car's computer calculates it based on what it measures.

I try to top up the tank fully after it shuts off but the handle won't let me add more.

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    Word of advice, don't let it get to 55 miles to empty. You want to have 2 hours of highway driving available at all times to properly react to an emergency!
    – corsiKa
    Mar 5 '18 at 17:55
  • @corsiKa, sure thing. I don't normally go below 80. But that's because I have a gas station on every corner and I normally use get-go where I get fuel perks which makes gas cheaper. So it's important to go as low as possible. The image above is from the internet, not my car.
    – Grasper
    Mar 5 '18 at 18:15
  • @Grasper what region do you live in? In Texas, we don't have "winter-blend" gas because it rarely even freezes here and in my life hasn't stayed below freezing for more than 2-3 days.
    – finleyarcher
    Mar 5 '18 at 19:36
  • @finleyarcher, Ohio
    – Grasper
    Mar 5 '18 at 19:57
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    @corsiKa where do you live for that advice?
    – plocks
    Mar 5 '18 at 20:04
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It is not likely the quality of the gasoline causing the change. The sensors are unable to evaluate the chemical makeup of the fuel. It could be a change in your driving habits, changes in temperature, the vehicle health, tire pressure, etc.

I would not be too concerned. The dash indicator is an estimate only. The computer monitors how you drive, current MPG estimates, and other factors to come up with that number. The reading is a reference only.

The best way to know for sure is to reset your trip odometer and compare it to the estimate. If you feel there is something truly wrong, it may be time for a tune-up and or diagnosis.

Winter driving has it's own challenges.

Fuel Economy in Cold Weather

According to the U.S Department of Energy's web site,

Cold weather and winter driving conditions can reduce your fuel economy significantly.

Fuel economy tests show that, in short-trip city driving, a conventional gasoline car's gas mileage is about 12% lower at 20°F than it would be at 77°F. It can drop as much as 22% for very short trips (3 to 4 miles).

The effect on hybrids is worse. Their fuel economy can drop about 31% to 34% under these conditions.

Source: https://www.fueleconomy.gov/feg/coldweather.shtml

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  • hm, maybe it is because it's winter. But I didn't have this problem last winter maybe because it wasn't so cold. Ok, I will wait and see if during the summer the estimate will change. Thanks!
    – Grasper
    Mar 5 '18 at 15:09
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    @CharlieRB depending on how the car calculates "miles left" it very well could be detecting difference in gas quality, albeit indirectly. Consider that "winter-blend" gas, as used throughout the US, tends to result in lower fuel economy. If the car integrates instantaneous MPG as many do, then it would definitely pick up the difference in the fuels if it were significant.
    – Shamtam
    Mar 5 '18 at 18:16
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    Weather is a big factor - my last car I diligently tracked my mileage, averaging 42+ mpg in summer and only around 36 in winter. "Winter-blend" gas does affect mileage some, but the cold air is a bigger factor.
    – sirjonsnow
    Mar 5 '18 at 18:29
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    @finleyarcher the air conditioning thing has been addressed by myth busters--they got the best mileage with the AC on full blast and windows down. While not exactly scientific (but they do try their best), the results suggested that AC has no appreciable effect on mileage.
    – phyrfox
    Mar 5 '18 at 19:05
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    @Grasper The quality and age of the engine oil seems to play a big factor, as does tire air pressure. When my car is freshly maintained, I get close to 70mpg in my EPA 40mpg car, but after a few months, it drops down closer to 50mpg. Regular, proper maintenance is a key to better mileage.
    – phyrfox
    Mar 5 '18 at 19:09
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"miles remaining" is an guesstimate based on your recent driving

The system has stores a great deal of data about your past driving performance, and the "miles to E" is assuming you will continue driving like you recently have. If your driving changes, the estimate is invalid.

Imagine you deliver vegetables for an artisanal farmer in upstate NY. Yesterday you drove to Vermont to pick up an irrigation part, all 55mph steady cruise, 32 MPG. You fill up the tank which has 10 gallons usable, so the computer says "320 miles to E" (32x10). Today you drive down to NYC to deliver to 20 restaurants, and get stuck in lots of stop-go traffic and city driving, that with need for heavy A/C drops your MPG down to 20. Finished, you fill up and the system says "200 miles to E" (20x10). The only thing that changed was your driving conditions.

And of course, on your 55mph trip home, that will prove to be wrong. The computer doesn't know what you'll do next.

How to compare fuel: standard test conditons

If you want to see what your gas is doing to your fuel econony, push the display button until it shows "instantaneous MPG" and set up some "standard testing conditions". A specific stretch of highway that's flat where you would be able to use a constant throttle setting to maintain speed (that means don't move your foot), A/C off, don't draft trucks, constant speed same speed because that matters, temperature should be in the same neighborhood (cold air is thicker) and reject any results involving rain or snow/ice on road because those hurt MPG.

What to expect

  • Expect E85 (ethanol) fuel blends to have significantly worse MPG - the fuel simply has less energy in it.
  • E10 fuel will be just a tick worse than regular gasoline.
  • Expect higher octane fuel to not help MPG at all.

It goes without saying (does it?) that the #1 thing affecting your MPG is your driving habits. To learn all best-practices there, research driving tactics called "hypermiling".

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  • I only use my car to go to work and home. Groceries during the weekends. My wife uses it for shopping. For the family trips, we use our second car(van). There isn't any variation in my driving. I noticed though that it was doing for my wife when she got gas. But I just thought she doesn't know how to fill up the tank. It is now doing it for me too. It was always around 420 for 2 years. It just happened now getting only up to 370 and it's been doing it for 3months. I'm very picky about how much my car eats gas and I have it displayed all the time.
    – Grasper
    Mar 5 '18 at 20:06
  • The car is automatic I forgot to mention but I thought it was obvious.
    – Grasper
    Mar 5 '18 at 20:06
  • @Grasper Maybe she's buying E85 by mistake. Indeed she may also be less than entirely filling the tank, maybe she stops it at a certain dollar figure etc. Or it may be winter: my car's MPG has also plunged since winter began.
    – Harper - Reinstate Monica
    Mar 5 '18 at 20:15
  • no, she is an expert. I'm from Europe originally, she taught me how to fill up gasoline. In Europe, we have different colors and type of fuel so I was confused when I got here. In any case, I think it is because of cold weather so I will wait and see if it changes after the weather gets warmer.
    – Grasper
    Mar 5 '18 at 20:20
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As this responds to the rate at which you've been burning fuel recently, give your car a general check-over: Tyre pressures first, but also check oil, and make sure you haven't got a boot full of junk. Also have you fitted roof rails or made any other external changes recently?

I used to monitor my fuel consumption quite carefully and had a fairly fixed routine. A deterioration often meant the tyres were getting a little soft. Now my routine driving is too short to tell me anything (I might not drive 50 miles this month) and long journeys are too variable.

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