Well you dont necissarily need to reset it for it to move. I figured you were talking about sitting at a light for 30 seconds. Lets say you have 1200 readings(20 minutes) of 17 mpg. Average in 360 readings (6 minutes) of 0 mpg or idle and you end up with about 13 mpg. If you had an hour (3600 reading) of 17 mpg driving you would still get dropped to 15.5 avg mpg.
And this was using an average of 17 mpg because I've heard of better and worse averages (and its pretty much what mine gets), if you went up a hill and had to give it some gas, that could lower the sample for those few seconds. Averaging that with a zero will make an even bigger impact.
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