Small update:
I installed some window wind deflectors which reduce buffeting significantly when the windows are down, or partway down. They also noticeably drop wind noise when the top is up, suggesting to me a drag reduction.
I took a road trip to New Plymouth and back for work. On the way up I had the top down, and targeted 100kph (62mph). My average was (adjusting for tire size, +1.7%) 21.1km/L, or 49.8mpg.
On the way back I filled with 100 octane and had the top and windows up, and averaged
24.8km/L, or 58.3mpg, also driving at 100kph.
On the return trip, it was cloudy, with some very light rain at times, and the temperature was between 20 and 24° C.
