Originally Posted by DrkKnight614
More often then not the read out on the dash changes due to conditions and will give you what your car is averaging at that moment. Its not necessarily what you average will be after a full tank, like what the other equation will dictate. Granted neither of these are 100% accurate.
Can't say I agree with that, but I'll explain why. My gas hogging 2008 4.6L V8 Explorer is a perfect example. If I fill the tank and never reset it, it'll read at or around 17-18 MPG the entire time. Even if I all out floor it a few times. It may change half a MPG or so, but that's about it.
If I hit the reset button, while I'm driving for example, I can be getting 25 MPG for a few minutes before the trip computer corrects itself.
In short, I'm sure that most trip computers are measuring average fuel economy over time. If it were averaging at the moment, my highway mileage or city mileage would reflect on the trip computer instead of my tank average. While I'm on the interstate I'm getting higher than my average, but that doesn't reflect on the trip computer, same for city driving. If I get on the interstate and it says 16.2 MPG, it won't reflect my average highway mileage for a while.
How long that time period it's averaging is unknown to me at least.