Cane it down a motorway at warp factor nine and your 40 'accurate' measure will go straight out of the window. Ride it around at 4k or less rpm in an upper gear and it could become 60, perhaps? Experience will tell you much more than a dodgy float meter and shonky electrics.
My 07 GSA OBC does just that, and I don't have to remember what the miles to light etc for that particular gear / revs / speed etc
