A
photo of the entrance to the 'route douanière' - i.e. access road to French sector.
There is a gas station at the
Leclerc supermarket on D35 2km before the Ferney-Voltaire border crossing, and another at the
Carrefour supermarket one roundabout past the Audi dealership (0.11km heading towards 'Gex').
Make sure to fill up in the section with a human operator at the exit booth if you don't have a chipped credit card of the sort accepted in French automated machines.
Not sure about normal gas stations; supermarkets sell the lowest-priced gasoline in France, for some reason.
IIRC - If I Remember Correctly
Before 9am itinerary C (D35>D884>D984>D1206>A40) is preferable to A, as it has negligible southbound traffic, unlike the Geneva ring highway, which will be heavily congested at the time, especially so in the next 6 months due to construction work on a tunnel.