Dont know where the 30mpg figure came from. Sounds like wishful thinking in my limited experience.
30mpg (US) = 7.8L/100km = impossible in any G-wagen ever.
I have one of 3 x om606 conversions downunder that we know of, and the best I have achieved is 17.5L100km (13.4 miles/gallon US) - this was the average on a 11500km trip, failrly loaded up.
This is not good.
I know of a friend getting 13L/100 in a 300GD (om617a) with a 5-speed manual.
But my injection pump has been from Australia to Sweden and back (dieselmeken, Goran) so we will see how the fuel economy goes when I get the G back home from the workshop. From the brief drive I had the other day with this tuned pump - the power was drastically improved (also from 5.5mm to 6mm elements)
However the old 4-speed auto was slipping a bit with the increased power so this week the 722.6 is going in...