I used to drive about in a daimler super v8 for a while (wasnt mine, just used to borrow it) fuel economy was PROPERLY shocking, like 15 to the gallon average and into single figures the moment you went anywhere quickly.
They are fast but not mentally quick, the gearbox is very slow to change down when you boot it and you really feel the weight when you are cornering it, its not something you can chuck about like you can an M5

Great cars, but only really as a cruiser not anything else, and the moment you use them for that, at 125 a litre and under 4 miles to a litre, it just becomes something you dont want to be doing at 30-40p a mile IMHO