Fuel Accuracy on the 777
Pitch Up Authority....According to Boeing, the FQIS on the 777 which supplies info to the totalizer (ie fuel gauges) has a maximum error of +/- 1% with full tanks, improving to +/- 0.5% for a near empty tank.
The fuel flow transmitters, supplying FMC info, have an error of less than 0.5% during cruise conditions. (this info in EK FAQ doc)
777 FCOM also states that displayed totalizer quantity at the start of the flight can be between 99% and 101% of the actual fuel. Calculated is set equal to totalizer fuel before engine start. (therefore, while calculated is supposedly more accurate throughout the flight, it is only as accurate as it was at the start..with totalizer's 1% error rate.)
At the end of the day, I'm not about to put on an extra tonne of gas just because the gauges may be underreading by 1,000kg. It does however open up an interesting debate about which fuel figure you should use towards the end of the flight if there is a discrepancy.