[Diesel_Dave]

I've been keeping a daily log of my commutes (40 miles each way) to and from work for almost 2 months now. I don't have a Scangauge, so for now I'm just using my in-cab display. It only gives trip mpg--not instantaneous mpg. At some point I know I should just give in and buy a Scangauge, but let's leave that aside for now. My question is about good days/bad days.

Some days I just seem to have an exceptionally good day or an exceptionally bad day for no apparent reason. By this I mean about 10% better or worse mpg than a "normal day". I keep track of the weather (temp, press, humidity, wind) and there doesn't seem to be any correlation there. I know sometimes I just seem to be able to catch/time most of the red ligths well (or sometimes when it seems like they all catch me), and there's always traffic variations that allow me to do more/less hypermiling.. It just seems like these variations I see are more than that.

What's somewhat strange is that it seems to show up early. By that I mean, sometimes I can just tell within the first mile or two, "This is going to be a really good (or bad) day!".

I guess I just wanted to know whether others have had similar experiences and/or figured out what's going on. I'm not really concerned about anything--just curious.