The W123 and W126 5-cylinder diesels were really not designed for very high speeds. Just look at their gear ratios. Turning 3200 rpm at only 70 mph certainly doesn't make it a high-speed car. 100 mph is about the maximum these cars can attain. If you want a high-speed diesel that's quiet, powerful and stable get one of the later ones, like the W124.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL