I think claims of 30mpg on a 300D come from either:

-urban legend
-inaccurate odometers
-poor math skills
-bragging/lying (whatever you want to call it)

Supposedly the 300SD will get better mileage. From my experience driving an SD I guess it comes from the fact that they have higher gearing. On the highway the engine spins about 500rpm lower for a given speed than my car does. This would definitely affect gas mileage.

Greg
'84 300D