i have a 1983 240d and when i go on the highway the car is really loud and my dad thinks the car isn't turning its last speed and says it has high rpm but he isn't certain since there isn't an rpm meter on the dash. so can someone tell me if thats normal or if theres a way to test it? Because i know diesels are suppose to run on low rpm.

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1974 240d
1983 240d -sold
1989 420sel with om606
1994 s350 turbodiesel - sold