If you’ve got both ‘wound up’ say on a motorway so to speak then the fuel consumption won’t be vastly different.
The 1.9 will have much less torque & power compared to the 2.8 therefore you will be making it work harder to achieve the end result which will affect fuel consumption.... hills in the 1.9 will be slower for sure. But it will get the job done, maybe not as easy as the 2.8 but the nature of the beast is not speed... much more gear changes with the smaller engine.
As for quantifying the costs, I would think in the scheme of things not much..... none of them do much better than mid 20’s unless a panel type van..... they’re as aerodynamic as a grand piano.
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