Hi JP, Welcome to forum. I dont have a Spark but seems pretty obvious from other Chevy model postings that the 'mpg to go' along with just about all other functions on the Trip including 'mpg achieved' is all very much hit & miss. The answer seems to be to use the odo set button at each fill up (or the old fashioned method of old envelope and pencil) and make a note of mileage and you'll get used to the approx distance your vehicle gives you per fill up.
Nice technology but in a word - unreliable!
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