My previous car, a Nissan Almera, was hard to fill to capacity on the level and would take over 2 gallons less when the side with the filler cap was slightly lower.
Used to park it on the wrong side when sloping towards the pump. Maybe this applies too to the 500?
Also cold weather, esp. cold starts, really ruin FE. If your commute is shortish that alone would explain the fallback.
Bet you will get 50+ mpg from that baby by summer
__________________
2011 Honda Insight + HID, LEDs, tiny PV panel, extra brake pad return springs, neutral wheel alignment, 44/42 PSI (air), PHEV light (inop), tightened wheel nut.
lifetime FE over 0.2 Gmeter or 0.13 Mmile.
For confirmation go to people just like you.
For education go to people unlike yourself.