Top speed for a 240D is 82-83 ish I don't recall exactly. Point is, you won't be doing 80 mph in a 240D.

Regarding mpg, your 300D might've been able to reach high twenty mpg if you drove it 60 mph. Same for the 240D, only low thirties mpg at 55 mph. Cruising at 80 mph in either car drastically reduces mpg to low twenties.

A w124 diesel would probably be more comfortable over the long haul. Equally so with a w210 diesel.

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1982 240 D, 308,000 - 321,127 miles (sold)
1982 300 TD,166,500 - 226,000 miles
1998 E 320, 120,000 - 144,000 miles
2005 C 230 K, 26,000 - 77,000 miles (sold)