[tremon]

I'm not sure that's always the case. Take a look at the PAL-V, which is in production right now (there was a short item on it on Dutch TV, they're currently looking for investors to boost international sales).

According to their specifications [1], it can do 6 km/l when flying, and 12 km/l on land. The specs they give aren't directly comparable sadly, so I had to do a short calculation to get the flying mileage (180 km/28 l), but it suggests that your 20x figure is one order of magnitude too high. The vehicle's range also suggests a 3x difference, not 20x.

[1] http://pal-v.com/the-pal-v-one/specifications/