How is fuel mileage calculated?

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Panshark

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ok, so I don't get newbie-flamed, I know how to manually calculate MPG. How does my instrument cluster compute my MPG on my 2006 XLT?

I topped off my tank yesterday at the only gas station around that sells unadulterated gasoline--no ethanol added (it's advertised at the pumps, it's the HQ for a local fuel jobber.) The fuel made the increasing-pitch whisper as it does coming up the fuel tank filler neck, and clicked off. I reset my trip computer, and reset my MPG calculator.

I was planning on driving 150 miles to my new home, stopping at a mid-point town to top off with cheaper fuel. I drove exactly 70 miles to the refuel point, refueling with 8.2 gallons. My trip computer was reflecting a healthy 15 MPG economy, however basic math shows that I pulled a sub-10 MPG on the first leg. I checked under the expy for fuel leaks--none. The rest of the trip home showed 16+ MPG on the dash, with an agreeing fuel gauge.

I'm planning on calling up the fuel jobber and asking them if there was a problem with their pump fuel. The additional details are as follows: I was loaded at 50% with boxes, grub and clothes. A hitch rack was installed, estimated 120 lbs load, grossing hitch weight at maybe 170. I refueled with 85 octane, which is the standard low-grade at these higher elevations. I did not pull any hills, keeping the speed around 65-70 mph. Overdrive was engaged.

I am vaguely aware of the newer vehicle's flex fuel capabilities, but having never driven in a metro area where flex fuel is pushed heavily, I've never filled my expy up with e85 or the like. Is a lying MPG calculator standard fare when using Flex fuels?

I am 100% certain that I did not accidentally fill up at the do-gooder e85 pump.
 
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