Rev cut - so you were bumping into the rev limiter?
Re: the 6L/100 km (39 mpg US) at 100 km/h - you said it was really windy. Think that may have affected the result? Head/crosswind on that portion of your drive?
EDIT: it would be great if we could get some speed vs. fuel consumption data for the car. Something like I made for my Firefly/Metro 1.0L when I first got it:
So I was seeing 46.75 mpg (US) = 5.0 L/100 km at 100 km/h. This was before I started seriously modifying the car.
FYI That was on bi-directional averaged runs on a flat road in calm conditions, warm weather.
View my fuel log 2014 Mirage base ES 1.2 manual: 54.0 mpg (US) ... 23.0 km/L ... 4.4 L/100 km ... 64.9 mpg (Imp)