Would someone please explain to me what big changes are expected to occur with break in that would significantly improve MPG? I mean what is the source of the big change?
In the olden days "break in" was a big deal because of the way engine parts fit and the properties of the lubricants in use. Special break in oils were common. When these were drained there was a lot of fine metal in them showing that the parts had been honed down.
Now, about the only concesion to break in oil is to run the MFG recommended lubricant AS BREAK IN OIL and then go to synthetic or whatever if you choose to do so.
I do understand the "vary your speed", "don't drive too fast", and "do not lug your engine" admonishments as parts wear to fit each other better, establishing good working clearances prior to allowing max stress. Note: I don't think it is likely you will "lug" a Prius.
What I don't understand is that, given the manufacturing tollerances in todays engines, what coild be the source for a really significant MPG improvement as the vehicle breaks in.
In a Prius, I suspect a good part, not all, but a significant contributor of this break in mileage boost, is breaking in the driver.
Another aspect of variable mileage: Someone mentioned cooler winter air as a source of reduced mileage because the air is denser. BUT increased air density INCREASES horse power output. It increases drag as well but my experience has been that the increase in HP has always been significantly greater than increased drag. Older hot rodders will recall that their "ride" always performed with more pep after a thunderstorm with cool dense air. Similarly, density altitude (a measure of air density) effects recipricating aircraft engines, reducing available HP with reduced density making for longer takeoff rolls at higher equivalent altitudes. Drag is reduced as altitude increases but acceleration on take off is not helped by reduced density because engine performance is the larger effect.
Intercoolers on high performance turbo boosted engines are there to cool the air taken into the engine.
I'm not saying Prii might not get reduced overall mileage at low temps, just that it isn't because increased air density reduced the engine HP.

Pat
