Most JLUs have a 21.5g tank, but not all:the JLUs come with 21.5G tanks, so I would think if the readout said 20 miles left (which roughly is 1gal) and you put in 17.5ish gal, that's about on par. 18.5 gal to readout-empty, leaves 2gal, which is about 40 miles reserve