I tried filling at half a tank before a couple of times in my Insight, but the mpg was all over the board versus the readout of my FCD so I went back to filling at empty.
Normally I try to fill up after I break 700 miles for the tank, which puts me somewhere between 9.25 and 9.75 gallons for the fill up with one bar left on the gas gauge. I fill at the same gas station, the same pump, at the same time of morning and fill to the second click, which takes away a lot of the variables you get using different gas stations, pumps, and filling at different times of day
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