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I usually try to get enough speed so that I can stay on low load low rpm when climbing the hill. Always that is not possible, so I try to stay on my very small rpm range where engine pulls ok, but does not go to rpm range where boost starts to come on (turbo diesel, it builds boost even when coasting if enough rpm is given).
If RPM is low enough pushing more throttle is not really helping anything, engine needs certain amount of rpm to be able to move car's mass and to overcome resistances and under certain RPM it might be able to do it at full throttle, but then again to move at same speed or bit slower on lower gear much less throttle is needed.
From many graphs I have seen that increase in rpm does not increase fuel consumption a lot, so using lower gear can be useful up to a point, it is not something you can always trust to be magical saviour however, sometimes I believe it is better to build up speed before the hill and let speed decrease while staying on low load and low rpm on top gear, this works best when you have many hills to climb, let downhill build speed, help with motor, then let speed go down at uphill. Problem is that sometimes hill is so big that speed decreases too much, then shifting down is best again.
I have only around 400rpm range where engine pulls fine and there is not much of any boost, higher rpm would work just fine, there would be power, but because of turbo I get more power than I really need, turbo boost triggers also boost enrichment which in mechanical diesel is set to safe side so it does enrichment plenty.
Limiting boost for additional 200-300rpm would give me better economy, but it is not quite simple thing to do.
Without turbo, I certainly would use lower gear with higher rpm so car would pull up hills more easily. As it is with diesels, they run at lean mixture, engine is getting all the time full amount of air, just amount of injected fuel is changed, so in that sense it is bit like lean burn, maybe also does get impact on fuel economy similar way, so that higher load is punishing more.
I have got very low consumption with way I drive, so there must be at least something that I do right, but as always is, there probably always is something more to be gained.
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