About 6 years ago Mark, a friend of mine from Cambridge bought a 4.3 V8 (2000/W reg Lexus LS430) from near Glasgow airport via ebay.
By foot, train, taxi and then plane from Norwich, he got to Glasgow airport early in the morning and was collected by the vendor.
He brimmed the tank and zeroed the trip meter and on-board fuel computer.
When he got home, he brimmed the tank again and checked the mileage on the trip and worked it out to 37.2mpg and confirmed the mileage against Google Maps which was within a mile or two. Given the fuel station was about the same distance away as the mileage discrepancy, he ignored it and was happy that his figures were reasonably accurate.
Checking the cars on-board fuel computer, that said 36.8mpg so that was reasonably accurate too.
Sometimes a V8 will in fact improve economy because it can be geared higher and generally gives the car a better power to weight ratio, for example the Rover P5 3 Litre (straight 6) was much more thirsty than the Rover P5 3.5 Litre V8 because the V8 was more powerful, more torquey and could pull a higher final drive and still outperform the 3 Litre.
That said, i would expect Bens S80 V8 to give better than 23.8mpg given it's a much more modern design than either of those already mentioned and is running a (theoretically) more modern engine management system.
Perhaps he likes the loud pedal a bit too much though!
Many moons ago when i ran a series of SD1 V8s, i regularly got ~28mpg no matter how i drove them including one "enthusiastic" trip where i covered 35 miles in 23 minutes. In other words, just because it's a V8 it doesn't mean it's thirsty.
