Your "K" factor must first be corrected during several top offs. After
that most of the discrepency is caused by various methods of filling to
various levels. Furhter, most gas pumps have cummy metering, and those
that are ripping you off are easy to tell with the JPI FF.

To be most exact, one needs to fill the tanks yourself, from the same
trusted pump. After that the JPI will easily be within .1 gallons of
actual use. Time and time again.

karl