Second, about 6-7% (Extremely rough estimate)
Boeing 777 data: (nmi = Nautical Miles)
Model Fuel Range nmi/g
777-200 31,000 US gal 5,240 nmi 0.17
777-200ER 45,220 US gal 7,725 nmi 0.17
777-200LR 47,890 US gal 9,380 nmi 0.20
Which means 238 to 280 nautical miles on a flight that is 3,772 nautical miles long. Which is a difference of 6-7%
I still wonder about the variation due to weather though. I can say that flight times vary by at least 7%, so that would suggest that fuel use variation should also be over 7%.
Couple of things to remember.
1. This was a single test flight. Not statistically significant by any measure.
2. My calculation was back of the envelope with unreliable figures. :)