I do reset at each fill up. After fill up, i start car, then reset everything before I leave the bay. I guess the computer takes my previous mpg and estimates the mileage range from that. I will try to reset when i first pull into the station before i turn off the engine, then fill up and see what the readout gives. Right now my mpg readout is at 47 mpg that equates to 940 miles. I doubt I can even get to 800 and don't know if I can take the stress and worry of becoming stranded. As far as the range going up and down, I noticed that in the city the range continually goes down and sometimes the mpg readout goes up, once on the highway the range starts going up and the mpg readout goes down. Kind of strange??