WTF, Sven?

So I've been aware that my '01 Volvo S60 doesn't exactly get the mileage I'd hoped when I bought it. Fuel efficiency was not at the top of the list when I made the purchase either...I had a limited budget, a short commute, and we were expecting a second child, so maximum airbaggage and safety came first—the fact that this car was really nice and I got a fantastic deal, cinched it...

Back to the mileage...First of all the tank on this car is huge. Twenty-one fucking gallons! (UPDATE: 18.5 gal.) So, with gas jumping up and down a quarter every other week, I never like to fill the car all the way up for fear of wasting five bucks if the price drops. This makes it difficult for me to keep an accurate count on the amount of gas I use. Couple that with the fact that I rode my bike to work frequently this past summer/fall, it was often weeks between trips to the gas station.

So here's my issue with this car. When I fill the car up, I try to remember to hit the old trip odometer back to 0.0 in hopes that I can figure the mileage next time I top it off (this never works because I never fill it all the way back up). But it often seems like I can go pretty far on the first quarter tank or so, and I think, "Hmm, twenty gallon tank, I've gone a hundred miles and it's still over three quarters full...not terrible." The next thing I know, I'm under a quarter tank and I've only gone another hundred miles...

What the fuck is up with that?

Last night I filled the car all the way up. It swallowed 14.595 gallons. The trip odometer read 206. That's an astoundingly Hummeresque 14.1 mpg. Now, that's in-town driving, including some early morning warm-up time, but still...that's awful. I had to drive up to Mrs F's grandmother's house and back that evening. Before getting home, I made sure to fill up again. 96 miles, 3.746 gallons...that's 26 mpg. Primarily highway driving, but an aggressive 75-80 mph. Not bad. but, that's a pretty wide split between city and highway. Or is the difference the first-part-of-the-tank factor?...Anybody else experience anything like this?

I'll be keeping closer track of this and following up. Because I know how fascinating this must be to all of you. Consider this an open thread to share what you current car is and what your mileage is, if you know.