The way I look at my manually shifted vehicles is that I control the acceleration via the shifter, not the pedal. The pedal needs to be as close to full throttle as possible, especially in a diesel. As long as I'm not going under idle speeds I upshift whenever I don't need more acceleration and downshift if I need more acceleration.
Granted I've mostly driven diesel vehicles with very low power to weight ratios. A sort of disadvantage to a turbo-diesel pickup is that for most scenarios they are overpowered. I drove my brother's Dodge Ram and felt like I was riding a bicycle.
Vroom!-clutch-Vroom!-clutch-Vroom!-clutch-Vroom!-clutch-Vroom!
I was also in top gear after about 40mph.
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