allright I filled the '87 300D up on level ground.
Using a flashlight to observe the fuel as it came up to the ring on the filler pipe I then set the trip odometer to 000000.
This time I took a couple of trips in the country, one for about 120 miles then a few days later I took a second trip to Napa and the wine country.
Then, when I was nearly out of fuel (well, my fuel gauge is acting up - another issue) I filled up back to the same level (Chevron diesel $3.499/gal - Yikes!).
Using the 328 miles and the 11.598 gals fuel added, I calculated 28.27MPg.

The trip was mostly freeway, maybe 10 miles in city traffic with some uphill climbs. I tried hard to keep my speed around 65 or 70 with an ocasional burst to 80 so I didn't baby it but I wasn't leadfooting it either, lets say conservative.
I'm happy with the figure. DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!