I think you need different bands of MPG for the V8 - like: below-15, 15-17, 17-19, 19-21, 21-you're lying!!!
Curiously, I averaged 20.4mpg for the last 4,000 miles before the 2nd service, cambelt change and new tyres (at 28,000 miles in December) and have averaged only 19.3mpg since... Can't imagine my 285 wide tyres all round help the MPG too much either...
I do moan a bit sometimes about the mpg (well, only when on company business) but I've worked out that none of my previous 9 cars did more than 30mpg (like NickW that includes an E34 540i Touring, an E39 540i Touring and an E61 545i Touring), no car I lust after or could contemplate buying would do more than 25mpg and I hate the dirt and smell of diesel, so I've convinced myself that 19mpg is acceptable after all!!! Besides, no other Touareg can provide the aural delights of a bent-eight, especially through the ABT exhaust... mmm!