Deanyel,
I would like to hear from other owners of the E400-420 series. My mileage calcs have been with multiple tanks over a long time frame. I know the V-12s get about 20 MPG on the open road. Jim Forgione's S-500 gets about the same mileage as mine. (from his website)
My E420 runs like a dream. New harnesses, thermostat (never goes over 100C), new air sensor, recent plugs, caps and rotors and O2 sensor. Running Mobil 5 (10?)-40 syn oil. and installed Jim Forgione's chip modification. Just got back from a 600 mile trip averaging between 70-100 MPH with an average of 22MPG. I have never had a Engine Light (except before the chip modification) in the last 3 years.
I find it hard to believe that you get 21 and 29 respectively.
Anziani

'95 E420 112K
'87 300E 203K