A couple of innocuous things could cause it (more) (archive)

(1) You changed to smaller (less wider) tires - I presume your original tires haven't lasted 8 years. Did you change your tire brand/size recently? Or have you been using same over last 8 years?
(2) Your tires were previously over/underinflated and now you inflate them to the right pressure - did you change your tire guage or the place where you get air? Some of these air pumps are notorious for giving incorrect tire pressures.
(3) Outside air temp! You get better MPG when it's warmer out. Maybe you'd been consistently looking at your MPG when the temperature was cold e.g. during winter months, and now you are looking at MPG when weather is much warmer? (A wild stab.)

I reset my OBC religiously everytime I refill with gas over the last 6 years. So I've come to notice small MPG changes from driving style, predominant driving gear, traffic conditions, octane ratings, hardware changes, tire brand changes, tire pressures, temp, etc.

If you have not been consistently checking your OBC each and every time, you may be unwittingly comparing apples to oranges.

Finally, you say that you'd independently verified the 18.1-18.8 MPG readings. Have you done same for new MPG readings? I'd be curious if that's indicating your OBC is just simply wrong.

: Around the beginning of the year, right as my 92 525ia crossed the 90,000 mile mark, I got an inexplicable increase in MPG.

: For the past 8 years with my mix of mostly in-town driving, I was getting 18.1-18.8 MPG overall. This was according both to the OBC and independent calculations.

: But since the beginning of the year, the OBC reports my average MPG in the 20.0-20.6 MPG range! That's with no changes in my driving patterns, no changes in the several places I usually buy gas (QT, Texaco, or Costco), and well between any times I had the car serviced. It started about two months *before* I took it in for its Inspection II @ 94K miles.

: While better MPG is a good thing, I am extremely curious how the car could suddenly start getting an extra 2 MPG over what it has been doing for the past 8 years. Y2K bug in the OBC? At 90K the engine is finally over its 'break-in' period? ;) Or what?

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