ive averaged 45mpg over a full tank before when driving from Wankum(the last shell station before leaving germny) and i never refilled had to refil just before honiton on the A303. that was about 580-600 miles.
was a night drive with zero traffic, and car was on cruise control @70mph, running shell vpower 100racing and bluefin stage 1.
its the one and only time have managed it. i nomally get drastically less than that