Bob - I don't know for sure, but yeah, I'd think staying around 80 would give you a big ding on your mpg.
As a comparison, I drive about 100 miles round trip for my commute, with about 10 miles of that in tight traffic. For the first 35 miles, I can go about any speed I want and for this last tank of gas, I set the cruise at 64mph.
For the congested 10 miles, I stay in 2nd as much as possible and go between 20-25mph. This way, I find that I can eat through most of the traffice waves and rarely, if ever, have to stop. It takes some personal discipline, and I'm sure I annoy some drivers who like to speed up, then stop, then speed up, then stop, etc.
For the last few miles, it's heavy traffic that moves, so I can go about 55mph.
I roll through as many stops as I can, things like that, and use my cruise control, both to maintain my speed and for small accelerations/decelerations.
That said, I did this over just the one tank. I don't normally drive this conservatively, although on my fastest day, I don't drive very aggresively, either.
Pool - the best I've ever had over an extended distance (80 miles) was well into the low 50's. That was on an interstate with no traffic, cruise set to 65mph. I think people see the EPA ratings and don't realize just how well this car can do if driven gently.