Back to my '83 300SD, the diesels have always left little to desire in terms of highway driving for me. I took that car literally all over the country, and it always ran nicely and smoothly. My '85 300D was a great highway driver also, though I didn't do near as many highway miles in the few short years I had her. My current '87 300D is the best of the three. I will gladly travel 200 or 400 miles to visit friends or family, and we always take *my* car!
Did anyone read the Car and Driver article recently about the most effecient cars on the market - it was Prius, Civic Hybrid, Echo, and Jetta TDI? There was a inset blurb in the article about diesels, commenting that the diesel driver is never bored because diesels ooze character. They ooze some other stuff too I guess, but character they never lack.

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'87 300D Turbo 220k (for sale)
'03 Dodge Sprinter 3500 tool body (shop truck)
'98 Ford F-150
'04 Subaru Forester 2.5X (wife's car)
'86 BMW L7 (for sale)
'69 Alfa Romeo Spider