We are also forgetting the effect displacement has on idling or other low load situations. Like the effect of cylinder deactivation. In the same gear, same Rpm, at idle the 3.0l will use close to half the fuel of a 5.9 given similar efficiencybin those situations.
As pointed out, there are times when too little power is worse than having enough. The RR here used to get F350 with 5.4l for MoW trucks until they realized that loaded down with all their tools and hi-rail equip the v10 actually did better on mileage because the 5.4l was having to spin higher rpm all the time.
If we can get 440/925 and 17mpg out of a 6.7, why can't we get 350/725 and 20mpg from a smaller but capable modern diesel? Plenty of power for all but the biggest of RV.
