Depends on car models, some are quite accurate.

But anyway, like I said it serves as an indication of how fuel efficient is your driving, the exact figures don't matter, the only things that matter is whether the figure is increasing or decreasing while you drive.

I already mention if you use instant FC you should only be paying attention to whether the figure is going up or down while driving, not the figure itself.

The avg FC computer on my car is fairly accurate since I also do manual calculation most of the time and compare the result to the car's avg FC display. It's not a Proton, though.