I found a Chilton Diesel book today at a used book store (published 1980). It refrences the EPA's combinded city/highway for the SD at 26. The 300D's EPA numbers were 23 city/28 highway.
So I should be getting better MPG.
The needle on this tank seems to be hovering much higher so hopefully it is just an issue of bad fuel from the filling station. Still curious though what the first lines of attack are when going after bad mileage.
eric z