Dear OP, does your car's winter range drop?
I can't seem to get over 280 during summer months and winter is even worse...about 240-250miles.
According to the car, i get about 60miles / kg, with the tank size at 5.5kg, I should easily have 330miles.
The dealer said thats normal of course and didn't want to troubleshoot the issue.
Thinking about it further, yes I get about the same miles per tank as you (~280) (though as I said I do not know if that drops in the winter). It is difficult to speculate how many miles a full tank of fuel would actually provide, since we cannot drive the car until it is truly empty. Have to go by the dash-display info, which may or may not be telling the absolute truth.
I chalked the lower number up to my particular driving style, and I assumed that if I drove more slowly and carefully (like a grandpa) my mileage would improve.
You've reminded me that I've been keeping a running tally of my fuel expenditures in Excel (as I have habitually done with all my vehicles--good diagnostic tool when I remember to use it). The mileage (miles per kilogram) jumps around a lot. Here is a graph with extreme outliers (artifacts I think of particular fueling problems at the West Sacramento pump on certain days) removed.
Does this at all jive with your findings?
The running average mileage (cumulatively summing up the total average mileage) is perhaps more interesting and informative:
This reveals a gradually dropping average mileage as time goes on. Intriguing. Assuming my calculations are legitimate, this suggests that the fuel cell and/or batteries gradually loose efficiency over time, which would not be mechanically surprising. Same sort of thing analogously happens with Tesla batteries, correct? At some point the fuel cell stack and/or batteries are toast and have to be replaced, I guess. Glad I don't have to cover the cost of that.
Still not sure I see any seasonal effects, though. Too much "noise" in the daily mileage data to discern seasonal effects, do you think?