I've attached a plot of my miles per kWh for my 2013 SV. I drive about 1300 miles a month (40 mile round trip commute + errands). I meter my L2 EVSE and I manually add infrastructure charging kWh on a monthly basis. The plot in blue is "from the wall" and the red plot is from Carwings (which also matches the car +- .1 mile/kWh, which I reset monthly). It represents a carefully collected data set.
Using carwings I get about 4.0 in the middle of winter and 5.1 in the summer and "from the wall" I get about 3.5 in winter and 4.5 in summer. There is a pretty strong correlation with the season. Efficiency has not varied much with age. I did notice an abrupt 3% or 4% loss in efficiency when I replaced the tires (OEM ecotopias to replacement ecotopias) but that did not really appear to persist beyond a month or two.
I have had not yet lost a bar and I have about 38K miles and 2.5 years on the car. The Pacific Northwest is a gentle climate for EVs. I find the efficiency differences surprisingly large given Seattle's benign climate. I use the AC and heat sparingly but I use the seat heater all winter. I do need to use defrost/defog functions frequently from Nov - March or when I have multiple people in the car.
I'm curious if others have similar data from other climates?