I've been using a ScanGauge with my Honda Accord Hybrid the last several weeks. Attached is an Excel file of just a few days of data I recorded with it. With the cold weather, some of the fuel savings features such as "auto stop" don't kick in until I'm almost done with my 6 mile drive to/from work. With the ScanGauge, I can see the instantaneous mileage, fuel flow rate, and other metrics. With regard to the instantaneous mileage, small variations in throttle position can result in major differences in mileage. With regard to the fuel flow rate, in 10 degree F weather the idle fuel flow starts out at about 1.2 gallons per hour. Once the engine is fully warmed up (after about 10-13 minutes of driving), the fuel flow at idle drops to around .2 to .3 gallons per hour. Thus, very short trips in cold weather result in poor mileage. On the one day with relatively warm weather (60 degree F) shown in my data, my mileage shot up significantly.
For those not familiar with the ScanGauge, below is a link to it. I am neither compensated nor affiliated with the ScanGauge folks.
http://www.scangauge.com/
I have the ScanGauge velcroed to the top of my dash front and center behind the steering wheel. The wire runs down the left side of my dash to the ODBC connector, which is located above your left knee when you are in the driver's position. During the day, the ScanGauge works well. However, I found that the orange backlighting on the buttons reflects from my front windshield at night and is distracting (looking much like tail lights on other cars). As a result, I turned off the backlighting which prevents me from using it at night. Maybe in the future I will build a little hood for the ScanGauge to prevent the reflection so I can use it safely at night.