You can't go by little short runs, resetting your mpg readout. By that method, if you want to feel really good about mpgs, go out in the country and find a nice long downhill run. Reset the mpgs and drive a mile or so giving it just enough gas to gradually get into high gear. Your mpgs will be up around 40 or 50 (or more).
Go fill up with gas. Reset your mpgs and your trip odometer and leave them alone until you fill up again. Drive "normally" for at least 200 miles and fill up again. Divide your miles traveled by the number of gallons it takes to refill your tank and compare that number to what the DIC mpgs reads. They should be pretty close, within 1/2 mpg or so.
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