Mine does the same thing with the IMPG reading (FYI its a regular old gryphon), and from what I understand, the way that the MPG's are calculated is by taking however much fuel is being shot out by the injectors and dividing that by how far the truck has gone based on the revolutions of the tires. Also the gryphon stores a few hundred thousand readings from what ive read (that is a guestimate, i forgot the exact number), which should last a few tanks to give you a better average reading. I have noticed that it changes quite a bit, so IDK how much that actually lasts with the readings, mine usually shows about 1-2 mpgs better than what my manual calculations are. I think the 40 impg readings would help keep this more accurate, but you will have to ask bill for sure. I hope this helps!
