Quote:
Originally Posted by Grady
Quit worrying about what the car display is telling you. Keep track of you distance and how much fuel you put in your car. This will need to be done over multiple tanks to get an average and reduce the errors of not filling up to the same amount every time.
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The gauge is reasonably accurate but will differ from the manual calculations since it is measuring mileage during more types of driving than the straight calculation. If you fill it with gas and just let it sit and idle until empty both the gauge and the calculation will read 0 MPG. If you let it sit and idle for half a tank and drive for 100 miles the other half the gauge will calculate an average based on the total usage which will differ from the calculation by a considerable amount since you do not calculate that gas used idling but the gauge does.
In this case the OP is plan and simple using the wrong gauge. The real time gauge will shot up and down dramatically but it is totally meaningless data. If left alone the average gauge will come surprisingly close to the calculated mileage. It needs time to set a stable average though. I never reset mine and it spends all summer parked firmly on 6.8. During the winter it slowly climbs to 7.4 before dropping back when it warms up. Now in fairness my driving pattern is almost precisely the same every single day so there is not much change but even if you do varied driving your average gauge shot not be shooting up and down. At worst it should slowly creep up or down.