If you set up a trip MPG via the X gauge, it gives you a running average that resets when the car is off for around 3 minutes. thus it gives you an approximation of your mileage on say your trip to work. that is handy when gauging how much better you do without having to do an entire tank.
it gives you a wider window into your actual mileage. the instantaneous mileage varies and bounces a lot... you'll see.
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