...Imperial Gallons then
colonel mustard: you could do an approximation just by putting a specific quantity of fuel in the tank and take the time the car takes to consume it. After that, you just divide both quantities and that would give you an estimate. Since I don't have a beetle yet, I can't do it myself.
The US gallon (known as the 'wine' or 'Queen Anne' gallon) is an older measurement than the Imperial gallon, dating back to around 1706. It was used originally for wine measurements, based around a cylindrical container 7 inches in diameter and 6 inches high. During the reign of Queen Anne, it was redefined as a box, 3 x 7 x 11 inches, which works out as 231 cubic inches exactly. This is more accurate than the use of 22/7 as pi, which is only right to two decimal places. The 231 cubic inch Queen Anne gallon is still used today, as the US Gallon. it is the standard fluid measurement in the USA, and in some central and South American countries, although most of them have gone metric now. A fluid ounce is 1/128 of a US gallon.
A US gallon is 3,785.412 mL (or 3.785 litres).
In 1824 the British defined a new volume measurement for ale, based on the volume of 10 pounds of pure water at 60 deg F and 30 inches Hg atmospheric pressure. This became the British, or Imperial, gallon and it is 20% bigger than a US gallon. The British abolished all other fluid measures, and the Imperial gallon became standard throughout Britain and the Empire (Australia, Canada, etc). A fluid ounce is 1/160 of an Imperial gallon.
An Imperial gallon is 4,546.09 mL (or 4.546 litres).
Australia ditched the old imperial system in 1974, and we have been a happily metric country every since, like almost all the rest of the world. The UK has been more reluctant to change, but they now also buy petrol by the litre, and the Imperial gallon ceased to be a legal unit in the UK in 2000.
Being metric, we use the standard Litres per 100 km to measure fuel consumption. Take your fuel total in litres, divide by the km travelled and multiply by 100. This is logical - if you use more fuel, the figure goes up. If you become more economical and use less fuel, the figure goes down. This is the opposite of the old, confusing, 'miles per gallon' (which gallon?), where the numbers went UP if you used less fuel.
To convert mpg to L/100 km is easy - you use a mathematical contant. There are 0.621 miles in 1 km, and there are either 3.785 or 4.546 litres per gallon. To get a constant:
A. US Gallon: 0.621 x 3.785 x 100 = 235.048 (you can just use 235 - it's close enough for normal use)
B. Imp Gallon: 0.621 x 4.546 x 100 = 282.307 (you can just use 282 - ditto)
OK. To convert either way, take you constant (either 235 or 282, depending on your gallon) and divide the figure into it. It works BOTH ways.
Eg1: 35 mpg (US): 235 / 35 = 6.71 L/100 km.
Eg2: 35 mpg (Imp): 282 / 35 = 8.06 L/100 km.
Eg3: 8.5 L/100 km: 235 / 8.5 = 26.6 US mpg
Eg4: 8.5 L/100 km: 282 / 8.5 = 33.2 Imp mpg
This is for normal fuel use in driving. VW 1200 Beetles generally used between 8 and 10 L/100 km, depending on conditions and how they were driven. More in the city, less on a country cruise.
Stationary use is another thing - that would be litres per hour. Volkswagen Industrial Engines may have had published estimates printed in their brochures, but I don't have any of those. Any figure given would also be an estimate, as these engines were designed for use over a range of engine speeds, constant or otherwise.
The only way would be to measure one directly. See how long it takes to idle away a known quantity of fuel.