I can't say about a DD bike, but I get around 15-18mpg out of my 800ss at race pace. The road bike does 40-60mpg depending on my mood at the time. I suspect the DD guys see not too dissimilar patterns of consumption.
Only ever measured the mpg of #34 one time, at brands hatch back in 2006. Was getting a 'credit crunch busting' 30mpg. But Brands isn't a fuel hungry track, unlike Donny and Snet for example.