Originally Posted by jimvw57
Well going by personal experience, the first 30K miles I averaged 38 MPG, after that it slowly increased to 42-48 MPG That was with recommended oil changes and easy driving. Not the only smart I heard that with
That's a very common experience among new car owners. I believe there are many factors, but the actual engine break-in isn't one of them.
But first, the engine break-in part. I own a Corvette and many owners say the engine doesn't break in for 20-30k miles. That was debunked pretty clearly years ago by someone who had opened up their engine with about 200 miles on it and did a microscopic examination of the cylinder bores, posting detailed and high resolution pictures, and then repeating the process about 10k miles in (unfortunately after a valve failure). There weren't any visible differences down to a resolution of a few microns. He tried to photo the same location in the same cylinder. Of course, there could have been other factors, such as a low spot in either the cylinder bore or the rings in that location, but the overall sheen of the cylinder bores looked the same as far as everyone could tell.
There's also articles like this recent one:
How to Break-In An Engine
which when talking about engine break-in and seating of rings, says:
With today’s rings, especially moly-faced versions, this can be achieved in a very short period of time and certainly within 20 to 30 miles of street driving. In WOT dyno testing, likely the rings are seated by the end of the first few runs.
As far as what causes fuel economy to go up over time, a lot of it is due to wear, especially in surfaces that mechanically mate under pressure like gears, and in seals. Also, as tires get worn, their rolling diameter decreases so the odometer becomes a bit optimistic, and tires get harder as well which decreases rolling resistance. Probably the main factor is that people either just stop enjoying the performance of their new car as much, or they get more used to driving it in a way that maximizes economy. After all, how many people do you know that complain about how the transmission in the 451 works, fighting it when they first get the car, but after a while they learn how to drive it more smoothly and efficiently? And I know from direct evidence that I don't drive my ED nor my Corvette nearly as hard as when they were new, and economy/efficiency have gone up in both cases.