Engine break-in and fuel economy

I’ve noticed my Yamaha YZF-R1 is averaging right around 43 mpg lately. A couple thousand miles ago (it has 7,000 on it now) it was getting almost 40 mpg. So fuel economy has increased even though I’ve been riding it faster and more aggressively than before.

Is this a function of the engine ‘breaking in’? (I’d have thought it would be broken in by 1,000 or 2,000 miles.) Or something else?

According to the Car Talk guys, engine break in isn’t really a well understood phenomena, manufacturers tend to guess (educated guesses, mind you, but still guesses) as to what the correct period of time. That, of course, doesn’t answer your question as to why the mileage has increased.

I can hazard a few guesses, however. One is that aerodynamics have a greater effect on fuel economy the faster you go. So, assuming you’re crouching down over the fuel tank when you take the bike over 55 MPH, and you’re now driving over 55 MPH more often (on many automobiles the break in period requires you to drive the vehicle below 55 MPH for a number of miles I assume there’s similar advice for cycles), and you don’t lean over quite so much at lower speeds, you’re effectively lowering your wind resistance. Another possibility is that because the rings have finally seated themselves properly, you’re getting more efficient combustion of the gas. Finally, it could be the deposition of carbon in the cylinders is increasing the compression ratio slightly, and thus enabling you to get better mileage.

I used to read Car and Driver.
They would post a car’s 0-60 time and 1/4 mile time at the beginning and end of a multi-month 60,000 mile or so test drive.
Every single article like that I read, the car would be faster at the end of the test than at the beginning. I seem to recall 0.2 to 0.3 seconds as being a typical gain in this process.