Mano mano is still the gold standard. As long as conditions are the same each time you fill up, that is, same gas station, same incline on the ground, etc.
The onboard computer uses MAF data to compute fuel consumption. It may not be as accurate as the mano mano method but I also don't believe that it could be way way off. I find that my onboard computer errs to the high side (more fuel consumed) as compared to mano mano, but not that much.
I also have an aftermarket trip computer called ScanGauge (
www.scangauge.com) which I plug to the OBD II port under the dash, and it pretty much coincides with the onboard computer reading.
I just think that moipogi's consumption of 6 km per liter is very high, let alone 4.5 km per liter. To think that this has been measured mano mano.