Q: My digital fuel mileage read out and is going up and down erratically

asked by on October 21, 2016

Two days ago I noticed my fuel mileage was at 17.2 mpg. I normally get between 16.5 and 17.5 mpg because I drive on the highway a lot to and from work. As I was driving to work two days ago the reading began to jump drastically so that by the time I parked at work the mileage read 22.6 mpg. I was only on the road for 20 minutes so I know that's wrong. But then the next day on my way to work it began dropping as drastically. By the time I got to work it read 13.7 mpg. I tried resetting it through the steering wheel controls but it didn't make a difference. The truck runs fine and I haven't noticed an increase in fuel consumption. What could be the issue?

My car has 60500 miles.
My car has an automatic transmission.

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Most "fuel economy estimators" utilize the length of time the fuel injectors are open (we’re talking milliseconds here) during each revolution of the engine while driving. As a result, small periods of acceleration or other load factors will have temporary injection length periods far greater than a longer term average.

So you get wide swings on fuel mileage as you observe. On some models, you have the option for averaging fuel mileage calculated on a trip, or have it average "instantaneously". The behavior you describe sounds more like it is using the very short term, instantaneous mileage calculation. Time to get the owners manual out and reset for a longer term statistic if that is more meaningful to you. If you would like to have this scoped out in person, a qualified professional from YourMechanic can come to your car’s location to inspect the mileage problem and follow through with repairs.

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