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You can't trust the mpg shown, since they tend to be off when calculating the average mpg (since they may not account for when the car idles, as it shows a blank "--.-" reading in those instances rather than 0 mpg, which is what is really what you get at that instant).
The best way to do it is to go by filling up the tank, driving, then calculating the mpg by taking the miles driven divided by the quantity of gas used to fill up the tank again. (It may not always be 100% accurate, but any variances will more or less get evened out.)
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