Nothing works well when over geared (i.e. in too high a gear) even a bicycle struggles to accelerate from low speeds in top gear..
For the sake of engine longevity, you should always try to keep enough revs on to allow your engine to perform well...
Flogging an engine in too low a gear puts severe strains on all parts of the drive train including the gearbox...
Diesel engines thrive on hard work, so give em bags of welly innit...
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