Fill your car up, record your odometer reading. When you need gas again, fill it up again, record your odometer reading again. Record how much gas it took to fill your tank again. Subtract the first odometer reading from the last, that is how far you went. Divide how far you went by how many gallons it took to fill your tank, that is your mpg. Use that number as a sanity check for your mpg meter. It should be close.
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