Thats perfectly normal, that very same gear can carry the car to over 120mph, so obviously its going to be only just above idle at 30-40mph.

Thats the beauty of diesels, they can chug along cruising at city speeds hundreds of rpm lower than a gas engine can. Makes them more efficient. Its where the extra *torque* comes into play. Diesels have a lot more of it available at lower rpms, a gas engine would sputter and die trying to move a car along at 1100-1200rpm/30-40mph.

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