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Old October 22nd, 2019, 08:47   #13
RIP TDI
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Join Date: Feb 2000
Location: Santa Barbara, CA
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Quote:
Originally Posted by oilhammer View Post
I have found over the years that the MFA's fuel economy trends closer to accurate the longer the trip is. So for shorter trips (less than 100 miles) it trends to 5-8% higher than actual, but for a 500 mile trip it ends up closer to 2-3% accurate. But it is always on the side of optimistic. Plus, it is going by the fuel measured by the ECU as being injected into the engine, not by fuel gauge position, although gauge position is what the MFA uses to calculate "distance to refueling". Which I always get a kick out of when you start out with a full, vented tank, because THAT value goes UP for the first ~50-100 miles.
Well, of course the ECU infers fuel flow from a variety of data rather than actually measures it, and that is the source of the non-linear error you mention.

My understanding is that "distance to refueling" uses inferred accumulated fuel used rather than gauge position for calculation, at least on the Mk 7. And that also explains the phenomenon you describe. The beginning value after a fill up is based on the calculated value from the previous tank and gets adjusted by inferred current fuel flow rate, which can be lower or higher than the average rate of the previous tank.
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