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Greater vacuum means that the engine is under a higher load (ie is choked down more), the greater the differential in pressure the more negative work performed by the cycle in order to draw in the required amount of air. Engines operate best/most efficiently (at least under acceleration) when they are un-throttled and operating near their peak torque value. This is not a likely value for steady state cruising when you do want to try and have the least air coming in as possible so that if the car is running properly at the stoichiometric ratio of air to gasoline (about 14.7:1).
The whole point of a downsized engine is to allow it to be operated nearer to wide open throttle/ the more efficient point in these curves. I've been trying to find one for the 4a-fe, but have only found the 5h-fe which isn't going to be the same.
In one of my hybrid and electric vehicle courses at university I've played around with data for a hypothetical Fusion extended range electric vehicle powered by a honda insight 1.0L 3 cyl but in order to do the evaluations in matlab it required square data so some of the data was outside the actual performance of the engine just to fill in the matrix, so plotting the wot values is critical to see where those points lie.
I know that when the engine is cold its probably better to leave it in gear because as soon as it gets taken out of gear the car shoots back up to its fast idle for warming up/emissions, where as in gear it might be at that speed, but sometimes I can actually be running it smoothly below 1500 but above 1k in gear (although I fear this may be lugging the car's engine abit)
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