Install a Scan Gauge to monitor all the parameters of your hybrid. Among other things it will show you the actual RPM, so you can tell when the gas engine is off. (
ScanGaugeII - Trip Computers + Digital Gauges + Scan Tool) I have mine mounted with velcro on top of the steering column. You have to crane your neck to see the odometer, but otherwise it is the perfect place. You will also discover that the engine rpm never goes to zero when the speed is above 42mph, even though the fuel flow may be zero (like going down hill). You will also see that the engine starts whenever you have the throttle above 2 bars on the KW gauge. In EV mode you can push a bit harder, but not much, and the EV mode always disengages at 25mph.
I use "econ" mode all the time and my overall mileage for 30K miles is 27.12 mpg. On my 2008 HiHy the on-board mileage readout is about 4% better than I get by calculating total miles/total gas used.