2015 Forester, manual transmission.

I drive to maximize fuel economy, and have reached 31.5 mpg. And there it stays, for a LONG time, even though by the usual progression it should have incremented at least a couple/three times now.

So - is 31.5 the maximum the computer will show?

  • The instantaneous display mode will show 99 MPG . I expect the trip mpg would also show 99 ; get to highway speed ,preferable with a down hill section of road, zero the trip MPG and take your foot off the throttle and coast until you get a refresh number on the MPG. t Apr 13, 2020 at 19:11
  • To get a real MPG , write down the miles driven and divide by the gallons of gas at the next fill. Apr 13, 2020 at 19:14
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    I think the overall figure is a moving average. If you reset it it will move fast for a while but eventually settle down at the average figure and change much slower.
    – Dave Smith
    May 3, 2022 at 13:46

1 Answer 1


Those displays are notoriously inaccurate, some can show 999mpg under some conditions.

Best method to confirm fuel consumption is to “brim” the tank and use that fuel to compare the distance when you refill the tank to the brim again for the exact amount used.

A friend checks the display against actual brim to brim values to find a regular 5mpg error but you might find different.

  • Many are pretty accurate when used consistently. I found a major station that had a bad meter once. When I went back the next day to complain , there was a heavy canvass bag locked over the pump ; The bag printing said " Illinois department of Weights and Measure." Apr 13, 2020 at 19:22
  • And , some years ago when there were still some locations that did not have alcohol in the gasoline, I consistently got a 10 % improvement in MPG display when using alcohol free gas. Apr 13, 2020 at 19:27
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    The instantaneous display mode will often show 99.9, but this is NOT the number or calculation I'm discussing. I'm talking about the CALCULATED OVERALL mpg number on the lower right as you drive, and that shows on top left when you shut down Apr 14, 2020 at 7:12
  • @RogerDavis So read paragrah 2...
    – Solar Mike
    Apr 14, 2020 at 7:15
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    Modern fuel injected cars will shut off the fuel in certain circumstances (going down hill with zero throttle for example) if its showing 999mpg when it does this it's basically because you are doing infinite mpg while no fuel is going in.
    – Dave Smith
    May 3, 2022 at 13:49

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