Is your speedometer readout correct? With the 35's and regear on mine I installed a speedo corrector first to dial in my speedometer as close to true as possible using GPS data.
After I did this I was hoping that I was going to get a proper MPG read out but still found it was off by around 8-12%. At the time I was piggybacking my O2 bank #1 signal into the O2 bank #2 due to a failing cat and a CEL and was thinking that this could be the reason for my miscalculated average. My thought was that maybe my injectors on the failing bank weren't putting in 100% correct fuel amount that the ecu was thinking.
Anyhow, after removing my O2 sensor fix and getting rid of my CEL I think the estimated MPG is getting closer to actual... As posted in the thread you shared, the MPG estimate you see is a long-term average and doesn't necessarily mean it will calculate out the same per every tank/miles driven. Only way to truly get that would be to reset your MPG every time you fill up and comparing the numbers again (granted your speedometer is reading 100% accurate).
