It easily fit purely two large suitcases and this can be finixieren well by the drawstrings.
The increased consumption by the box of course depends on the driving style and speed. It took us longer to 100km with the Touran about 1 liter and with the Alhambra about 0.5 liters (both TDI). The at 120-140 on the highway. At Alhambra (identical with Sharan) they can be aligned by the size of the car more or less perfect in the air flow of the windscreen which was filed with the Touran not quite as good, thus also the higher consumption.
All in all a good buy and recommended.
