Paralyzer

I recently got a Rinda Tech Mate Pro. When connected to my 05 Mercury Racing Scorpions it reads fuel flow in GPH. I didn't know it would do that. My motors use the older GM-Delphi MEFI 3 system. I thought fuel flow was only available in the newer systems. Does anybody know how accurate these reading are? If they are close then that 1 feature could pay for the thing quickly in fuel savings when I figure out my most efficient cruise speed.