If you want to get exact readings of your mileage be it instantaneous, trip, tank, long term, life time etc, get a scan gauge. We use one on our smart car diesel and by watching the instantaneous it can help you improve your driving style etc for max mileage. We get low to mid 50's in the city and mid 60's to low 70's on the highway if we keep it under 65. 65MPH and above nets low to mid 60's for mpg (US mpg). Diesel and gas are pretty much the same price now in Palm Springs. 3.89 regular, 3.99 diesel.
I like the VW. If we ever need 4 seats again I would consider it although we would probably go electric next time.