If the flow is upwards over the rear windows, could it be creating trailing vortices?
May i propose bringing the rear deck up with something easy to make that wont obstruct you view:
__________________
2016: 128.75L for 1875.00km => 6.87L/100km (34.3MPG US)
2017: 209.14L for 4244.00km => 4.93L/100km (47.7MPG US)