I don't know if this will help anyone out there but if you want to make considerable savings then here's how we do just that.
I'm afraid that, with advancing years, we just tend to set 1400rpm and take what speed comes (unless it gives me over the speed limit, of course). Typically, in still water that will be about 5mph.
If you look at my fuel graph for our Nanni 50 (which is a pretty typical power unit) you'll see that we also make a fair bit of savings!
If I get Breydon wrong and have to go up to 1800rpm, or Heaven forbid, 2000rpm, the resultant fuel use/cost is horrendous!
On displacement boats you seldom go any faster than the design speed (Between 6 & 7 mph for most Broads motor-cruisers) but just put up a bigger bow wave and wake.