Here's how much in-town driving messes up my FE:
 

I have a '98 Explorer, 4.0 liter SOHC. All stock except for an extra 100-125 lbs of stereo in the back (soon to be downgraded to about 50 lbs). I think my Explorer is EPA rated at 14 mpg city, 20 mpg highway.

My route to work is 26 total miles, 21 of which are a speed limit 55 highway and the other 5 miles are in town. The highway is completely hilly and curvy, I doubt I could go 10 feet without a change in elevation (and you thought Kansas was flat!) or 100 feet without a bend in the road. I use a constant throttle approach for the most part, except on a few hills where not letting off would get me speeding too much on a downhill (above 62 mph is my limit) and not pushing it will make me upshift on an uphill (this happens at about 45 mph).

For the past two months I've filled up once a week, so my total mileage has always involved at least 2-3 forays around town for errands and such. But still, my mileage remains respectable at around 19.5-20 mpg (including my trips to work, this comes out to about 75% highway, 25% city). Not bad.

But this week I filled up on monday, haven't done any driving around town all week, and filled up today after work. So it was all work driving, and I got 24.5 mpg! I'm pretty happy about that.

Oh and I forgot to mention that I'm going to do some runs using my cruise control so that I can find out how much constant throttle helps.

Cruise control makes constant speed, not constant throttle.

I've been wondering how constant throttle ("DWL") works. People report good results with it, but I don't know the theory behind it. The only thing I can think of is that average speed goes down...

I know cruise control is constant speed, I'm figuring that constant throttle gives better fuel economy. So I'm going to test and see how much the constant throttle approach helps over cruise control.

Average speed shouldn't go down. If you go up a hill at 45 mph and go down the hill at 65 mph, shouldn't you be averaging the same 55 mph? For me, I figure at least not downshifting while going up a hill helps (like I would if using cruise control), if nothing else. Though, granted, it's actually fairly tough to keep a constant throttle all the time. I haven't noticed a longer trip time, so I don't think I'm going any slower overall. I think I would need at least a 1 mpg improvement over cruise control for me to say it's significant though. Not sure if I'll get that, but we'll see.

Problem is that you will lose more going up than you will gain back going down. It takes patience... thats for sure. I've done a little of the DWL.

Some advice, if you are coming to a up and down hilly section...use slightly more throttle when going downhill, and then back off and do the DWL when going up... That way you gain momentum, use gravity to your benefit.