In the UK, the councils' trading standards departments are responsible for checking filling station pumps. They carry out random spot checks using calibrated equipment and the margin of error allowed is +1% and - 0.5%. It's very rare you hear of a filling station owner being taken to court, simply because they have no idea when they will be inspected, and the fine will remove any profit they might make from cheating.
From Aberdeen trading standards website FAQs:
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I recently put £10 of petrol in my car but I don't think I got it all, what do I do?
In the first instance take a note of the pump number and call your local Trading Standards Office, they will send an inspector to check the pump. However modern pumps are very accurate and problems are relatively few, in fact meters have changed very little over the past 20 years or so because of their robust design. Typically petrol pumps will be accurate to at least 0.3-0.5 percent
And sometimes those spot checks can help garages stop giving away fuel
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you also have to have a 240V power feed. It takes 4 hours to replace the gas you'd use to drive 50 miles
That sounds like a nice way to lessen one's carbon footprint. 
Somehow, compressing natural gas to whatever pressure it is using in the car, especially compression that requires 240V power, gives me more than a little uneasiness. Have you ever seen a bottled gas tank careen around someplace after the neck gets broken off. Two major problems with this setup, IMHO. First, the tank must be extremely safe which means heavy which means dead weight in the vehicle. Second, the pressurized gas can be very dangerous should the tank suffer even a tiny leak. If it doesn't, literally, propel the car in some random direction, it will be an ideal blow torch looking for any spark!
But I'm sure we can trust Detroit to take care of all those little nuisances before selling them to us...
Have you ever seen a bottled gas tank careen around someplace after the neck gets broken off. Two major problems with this setup, IMHO. First, the tank must be extremely safe which means heavy which means dead weight in the vehicle. Second, the pressurized gas can be very dangerous should the tank suffer even a tiny leak. If it doesn't, literally, propel the car in some random direction, it will be an ideal blow torch looking for any spark!

Jim, I have the same concerns. The article did mention that the tank is, well, a tank - heavily reinforced, and that they had studied temperature extremes extensively. But, as the song says, "It only takes a spark - to get a fire going...