Re: Who's had the harness installed?
For me, cold weather seems to be the biggest factor for mpg. Since my typical drives are 30 min or less, the cold hybrid engine doesn't do anything for mpg for a large portion of every trip. Typical mpgs in winter are mid+ 20s, and high 20s for extended drives.
With warmer weather, my mpg goes up a lot. I get regenerative braking, motor assist and autostops within a couple of minutes. My mpg is high 20s to low 30s, and occasionally mid 30s for extended drives.
Driving technique probably plays some role. I stay smooth and steady on the gas pedal once up to speed, and keep the eco light on. The manual says 45 miles per hour is best, not that I have much control over the traffic around me. Other postings describe the best acceleration to be smooth moderate, not granny. When slowing down, I coast to lights if traffic allows to collect max braking energy. I think this makes more hybrid energy available to do light battery assist during cruising to save engine gas use.
When using AC, I switch to recirculation with the green AC light with the right vent position knob set to the first two positions. The manual (page 156) says the AC is used if the right knob is in the floor mode as well as the defog mode. In defrost, the engine autostop is disabled. Somehow, the recirculation is mixed up in all this.

I have just spent more time reading the manual than when I first bought the car.