Cold, cold, cold
It's been extremely cold where I live lately -- as low as five degrees in the morning, with highs in the low twenties (Farenheit). This had a big impact on my gas mileage.
I blocked my radiator again (I'd tried once before an odd way that didn't work), and found very shortly that it had a really nice impact on my mileage while not creating any discernable effect on max engine temperature -- just how fast it gets there. I stuck a large chunk of cut-out cardboard in front of the radiator, covering all but the top one inch. There's even a nice ledge to park the board on! It kind of makes the front look a little funny if you peek at the "smile", but it works well. Plus factor for me is that the heater warms sooo much faster. I used to be at least six miles into my commute before the thermostat was at six bars, now it's down to about 3 (the first leg of my trip in the morning is all downhill, so there's not much gas getting burned). I wonder whether it would continue to be safe driving if I covered up that top 1" of radiator too, and just relied on wind leakage to provide air...
Anyway, just that one mod (plus a gentle right foot) kicked my personal MPG since I bought the car up from 54.9 to 57.1. My daily commute is about 40 miles each way. The first leg is down a slight downhill for 12 miles, then 20 miles of flat, then 8 miles uphill a bit again. In a way it's nice, because first thing in the morning my car does that "forced charge to warm up the batteries" thing almost every day lately, and when it's only about 70% up anyway, I avoid the symptoms of "full battery, no more please".
The fastest speed on my commute is 65MPH on freeway for the last 15 miles or so. The first and second legs are both 60MPH the whole way, usually down to about 50MPH for several miles at a time due to traffic, and with some stop-and-go right before I merge onto the 65MPH freeway. I've been really impressed that the EPA estimate (if I block my radiator in the winter) is so dead-on with reasonably careful driving.
That mileage was almost all at factory-stock pressures, and my lifetime MPG has not budged yet from 54.9 at 24,800 miles when I bought the car. I rolled over 27,000 today. My wife prefers to drive my car into town because she loves the gas mileage and sporty, in-control feeling versus our big 1994 Mercury Villager van.
I just re-inflated the tires to 50psi on each corner because, although it's been bitterly cold and foggy, it looks like it's breaking today, and I'm eager to see what my mileage will be like in the morning. My wife also has a bit of an insensitive foot for the Insight, so when I get it back from her my per-tank mileage is generally down a few MPG

Just filled up, so it will be fun seeing how well she does on the video game tonight as she drives to a meeting! She told me she'll try to play for a high score like I do from now on...
So, to sum up:
Lifetime MPG: 54.9 @ 27,000 miles (though I've only had it for 2,200 of that), car always driven in cold climates (Spokane, WA and Tooele, UT). Primarily driven in the city prior to my purchase, primarily driven on highway at slower highway speeds since I bought it. Except for my wife, who drives on roads where people routinely do 80-90MPH...
Personal lifetime MPG since I bought it used: Now 57.1 with radiator block at stock tire pressure. I suspect my personal will be somewhere around 61-62 as I slowly bring up the LMPG. Or maybe I should just reset the LMPG now
Goal MPG: 60 MPG. I think with more practice, and harder tires, 60 LMPG is an achievable goal over the next 100,000 miles. Some people get over 60, but in a cold climate with lots of ice, snow, and slush on the road frequently, unless I decide to reset the LMPG, I consider getting over 60 really unlikely for me.
OK, maybe I should just put all this stuff in the Lifetime MPG database
