Does the mileage indicate the quality of the gas?

Question

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:

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.

Answer 1

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

Answer 2

"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".

Answer 3

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.