The most I ever put in my Vauxhall Cav was 175 litres. I'd say you need to be very careful if you are considering carrying 240 litres. I got round the weight issue by buying a 15 litre DI vessel and if I needed more water when I was away from home I made it with a custys outside tap.
However, Id imagine and wouldnt be prepared to put it to the test that 240 litres of water will have different physical characteristics than 2 fat blokes sat in the back of your car, going round roundabouts will be a nightmare let alone anything else.
I was going to try to elaborate how I understand it, but basically I think 240 litres of water is going to be bloody hard to drive with, safely, by comparison to a couple of people.