Following over a decade's discussion on this, it's apparent that 75/ZT OBD computers were calibrated over-optimistically in production. If you want better accuracy, it must be set correctly for the figures to reflect reality. The addendum to my Diagnostic Routine write-up shows you how to recalibrate it. But realise that significant variations in your driving style will skew the results, as will a change of driver. Above all, never reset the system after calibration or you'll lose the benefits of the computer's 'experience' - for want of a better word.
HowTo:
https://www.the75andztclub.co.uk/for...ad.php?t=55585
TC