20 highway mpg is on the very low end, I would expect more and begin troubleshooting (beginning with examining your driving technique to see how much of a factor that plays into it before spending any money on it). See if you can find a mechanic who is experienced in Bosch CIS-E injection systems, MB guys and "foreign" auto repair specialists should be fairly well versed since Bosch stuck these on a lot of different cars over the years.

For the record, I regularly meet or exceed 28mpg in my 300E on the highway, its definitely not unrealistic. Mileage on my tank yesterday was 29.2, mostly highway driving. Flame suit on.

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth