Good posts.
But the original question isn't really practical. Fuel economy is best measured "over time." Measring it one gallon and/or one hour at a time isn't practical and may even be confusing (the "is ICE warmed up" question is a good case-in-point).
So I will tackle this one in general terms, avoiding the "one gallon/one hour" question entirely.
I have observed that high speeds (over 55 MPH) cause the biggest fuel economy penalty. More on this later.
Stop-and-go traffic, in my opinion, causes the second most siginificant penalty, even after you account for regenerative braking. Yes, this is contrary to EPA estimates for the Prius, but we all know that the EPA isn't really that "real-world" accurate for most cars anyway.
My logic (based on my observations and lots of time behind the wheel):
My best fuel economy is between 35 and 50 MPH, depending on grade, wind, the weight of the vehicle, passengers, and cargo.
As we already know, when going below 42 MPH, the car will be more willing to alternate between ICE and electricity, or a combination. My car will not usually go to electric-only/ICE-off "stealth" at any speed above 40 MPH, unless my SOC is at 6 or 7 bars (highest blue or lowest green).
Regardless of whether or not the ICE is running, I can drive between 35 and 45 MPH and consistently keep the instant display steadily greater than 50 MPG (after initial accelleration). That's my "feelgood" point; it's a happy medium between being a slowpoke roadhog and a resource-conscious driver.
Sometimes, the instant will float way up there in the 70's, 80's, or 90's, even with the ICE running! The key is to drive at as consistent a speed as possible.
But here's the crux: You pay a HUGE fuel penalty whenever you need to accelerate. Yes, regardless of what's already "in" the battery,
it still takes gas to accelerate. I often see my "instant" go down to 20, 16, or 12 MPG while accelerating.
For the length of time it takes me to get back up to the speed limit, that is a penalty, and it mathematically works AGAINST all of that good "at speed" driving where I was getting MPG figures in the 60's to 90's.
So yes, stop-and-go traffic still penalizes me. I have observed my best mileage to be where I can drive no less than a quarter mile or more between slowdowns (for traffic turning off the road in front of me) and stops (for traffic or red lights).
If I can go a half-mile at a time, that's even better (mathematically speaking), because I am accelerating only half as often, for only half the time.
AS I mentioned earlier, high speeds are also gas mileage killers. My average gas mileage when I stay off the freeway is between 49 and 55 MPG.
When on the freeways, I try to keep between 55 and 60 MPH, but often I am forced to speed up just to avoid being killed by tailgating H2's and Escalades.
When I drive on the freeways a lot, and especially when I make long trips, sometimes I will go as fast as 70 to 80 MPH to keep up with traffic. This is a really big "whack" in the gas tank, and it reduces my fuel economy down to something between 43 and 49 mpg (depending on speed).
Here are my latest observations, based on about 11,000 miles of driving with my car, assuming a light load (me and my schoolbooks and other junk), reasonably level terrain and light to moderate wind conditions (0 to 10 mph from varying directions).
At speeds below 50, I can get anywhere from 49 to 55 or more MPG, depending on how often I have to
accelerate.
At speeds above 50, I get between 49 and 50 MPG.
At speeds above 60, I definitely get less than 49 MPG.
At speeds above 70, I get significantly less than 48 MPG.
At speeds above 80, the wind-resistance penalty is quite pronounced, and I get less than 43 MPG.
Notice the "curve" gets sharper (more rapidly decreasing fuel economy) as I go faster. I suspect that this is true for any vehicle. On Earth, anyhow....
My overall fuel economy is around 48 MPG, and my car is in the database at
http://www.greenhybrid.com .