Pulled the glow plugs on my 4sp 240d a few weeks ago and found healthy plugs but fairly coked holes. Reamed them by hand using a variety of drill bits, reinstalled and voila.
25/26 mpg has become 30mpg (after 3 tanks).

I'm inclined to believe it's a fluke but I've controlled for driving style, distances travelled, fuel type, etc. This seems to be the only variable.

Has anyone experienced similar results?
And if so, can you explain the mechanics behind the effect?