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I find that the factory MPG displays are not very accurate.
First, you need to see if the speedo is calibrated properly. On the highway, compare how far your car says you went vs manually counting road markers on the side. Do this over a 5 or 10 mile stretch to get a good idea of how much % it may be off. Once that is sorted, reset your trip meter and record how many miles you go until you have to refill. Divide that number by how many gallons were put back in + your speedo error, and that is your actual mileage.
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