As promised:

Car is 1983 300CD. Fresh oil change with 15W-40 AMSOIL. Fresh valve adjustment. Fresh air filter and fuel filters.

Starting miles: 290788. Ending miles: 293341
Total miles driven this trip: 2553

Trip route:
Leg 1: Near Richmond VA to Tampa, FL via U.S. 1 to U.S. 301 (no interstate hwys)

Leg 2: Tampa to Ft. Lauderdale. No interstate hwys
Car parked in Ft. Lauderdale for 5 days.

Leg 3: Ft Lauderdale to Charleston, SC. No interstate.

Leg 4: Charleston SC. to Home. Interstates all the way at speed limit (65 to 70 mph).

Air conditioning most of the way, cruise control used where feasible. Ran speed limit. Did not avoid small towns.

Fueled 10 times (kept at 1/2 full).

Threw out the high mileage and the low mileage and averaging the rest provided a real world mileage of 29.456.