ECU change vs computer readouts

Folks,

Does any change to the ECU make the computer MPG readouts go haywire?.........have fitted a higher BHP/torque ECU and now the mpgs (average and instantaneous) are inbelievably high (not borne out by the old fashioned mpg method)!

Is this 'just the way it is' after an ECU change or is there a tweek to make it read almost accurately?

Thanks