It is confusing when you relate personal Australian experiences, with what you read in American magazines. The measurments are different and cannot be compared! Before 1974 Australia used the Imperial (British) gallon, which was 4.546 litres. The Americans, on the other hand, use the US gallon which is only 3.785 litres. When you refer to '26 or maybe 30mpg' and '18 mpg', which one do you mean?
If we stick to modern measurements then there is no confusion.
Torsion bar Beetle fuel tanks are 40 litres. If you want to drive an arbitrary 500 miles (804.65 km), then you would need to achieve 4.971 L/100 km. VWs generally average between 10 and 11 L/100km around town, and perhaps improve to 8 L/100km on the highway. The best I ever did in my '64 1200 was 474 km on one tank, on a drive to Young from Sydney back in the '80s, which works out at 8.44 L/100 km. I was cruising at 100 km/h, so you could probably do a bit better by keeping below 80, using a feather touch on the throttle, coasting or even shutting off down hills, etc etc. You might get into the 7s, and perhaps into the 6s. But I think getting BELOW 5 in a Beetle is impossible.
It IS possible in a modern VW. In 1978 the Golf Diesel (1.5-litre) was introduced to Australia. It won the Total Oil Economy Run that year, averaging 4.4 L/100km. Kevin Bartlett also drove a Golf Diesel from Sydney to Melbourne to demonstrate its economy, and averaged 3.62 L/100 km.
The Golf 3 Umwelt diesel was the first production VW capable of more than 1,000 km on a standard tank of fuel.
Today's Polo 1.9TDI averages 5.0 L/100km and is capable of up to 900 km on one 45 litre tank. This is the most economical VW currently available for sale in Australia. I drove one for a week last year, and it was a brilliant little car. 74 kW and 240 Nm gave it fantastic performance, faster than my old 2007cc Beetle ever was. And it isn't even the best VW model. In Europe you can buy a model called the Polo BlueMotion, which is VW's ultra-clean 'green' model. With a 60 kW 1.4 TDI engine, it averages 4.9 (urban), 3.2 (extra-urban) and 3.8 (combined). There was one on display at the Sydney Motor Show last year, but sadly it hasn't been released for local sale. Pity - if it was, it would be the most economical car on the Australian market, as well as the greenest (CO2 < 99 g/km), both better than the Toyota Prius.
http://www.volkswagen.co.uk/#/new/polo/which-model/engines/fuel-consumption/