Economical speed, W210 turbodiesel

[peapod]

Best economy comes with engine efficiency and one assumes the boost pressure should be at an optimum. My car (auto turbo diesel ) seems happiest at about 2400 rpm 75 mph in 5th, at 60 one needs to downshift to recover liveliness. Can anyone advise on whether one should shift to 4th gear at 60, on the flat and at steady speed, to increase the revs and boost pressure. Would that inporve the economy. ? Or is the boost pressure a function of torque and fuel input? Would the car me more economical in 4th at 60mph than 5th at 60mph. ??

 

[Supacool1]

5th as you will using lower revs to substain motion. You can use the torque of the diesel to recover liveliness. I read somewhere that to get the best possible fuel economy you must drive within a speed range of 50-60mph to attain the best MPG in top gear.

 

[Bobby Dazzler]

Shift up as soon as speed is sufficient to make sure that the RPM in the next gear is in a "comfortable" range, ie not labouring. Generally higher gear and lower speed maximises fuel economy providing that speed is constant and the engine doesn't labour. So for cruising at the speeds you mentioned, you really ought to be in top gear.