20mpg is done in a lab under controlled conditions, which you won't be able to reproduce...If there's nothing mechanically wrong, causing the low mileage, then your foot and necessary driving only is the only way you are going to see any increase in efficiency.
My SEL is advertised for 18mpg I can really only hope for maybe 16...IIRC a 00 Mercedes gasoline sedans made it onto the cash-4-clunkers list here in Canada..

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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