Optimum diesel economy - Andrew-T
Given that a diesel engine gives max torque around 2000rpm, is it more economical to use 4th gear for cruising at 40-50 (e.g. on a speed limited road) than 5th, where the engine is only doing 1500-1600 ? Or will there be little difference?
Optimum diesel economy - googolplex
Not an easy question to answer unless you run a lab which tests this sort of thing, but then, as Ford found, even the lab testers seem to have some wayward measuring appliances.

My gut feeling is that you are right and that 4th is just as good at that sort of speed. The seasoned advice, which I cannot wholeheartedly agree with, is to stick a diesel in top gear asap. If my fuel computer is anything to go by (and there's another wayward measuring device) anything much below 1600rpm is going to lead to increased mpg particularly when accelerating.

Splodgeface
Optimum diesel economy - Ben79
The trip computer on my C5 (and the same readings on the Picasso HDI) suggests that economy is best at around 1500 rpm, performance isn't so great, but it is only 250rpm from max torque.

I guess this is because the turbo isn't up to full boost, and you are driving using more torque.

Ben
Optimum diesel economy - Altea Ego
I have tried this using trip computer instant read out. (well after 1/3rd mile) Ok its not accurate but its a releative indicator. Indeed the sweet spot (on mine 2k reves on the button) is the place to be re consumption. Lower revs/higher gear uses more fuel.

Ok it may not be the same on every car, but thats true for mine.

 

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