Well, it's almost exactly 200 miles from my home in Alexandria to my parent's home in Virginia Beach. My 5-speed NA 84 TD has an incorrect replacment speedometer for a turbo model. So, using the interstate mile-markers to calculate the correction-factor, on my last trip I got around 30 mpg highway, with the AC going, at an average 65 mph. This includes the few miles in local traffic when I left Alexandria and just before I arrived in Virginia Beach.
I'm hoping to make the same trip next week and retest, hopefully without needing the AC this time, as it needs a new LP hose and freon now, and after some brake work, since lately my front brakes have been dragging.
Unlike the typical eBay W123 peddler claiming 40+ mpg, 30 is not an outrageous figure for these cars.

Happy Motoring, Mark

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DrDKW