Well, it's Labor Day, the traditional Last Day of Summer in the USA. I've kept track of my electricity usage and trip distance all summer. Here is the result in graphical form (the blue bars are the distances of each trip; the red lines are the efficiency of each trip):
Unlike the earlier data, there is very definitely a correlation between trip length and efficiency. I'm measuring the actual electricity used to charge the vehicle. Plotting the data another way, and fitting a least-squares line to it, shows this correlation very well:
Basically, you can model the driving costs as 379 Wh/mi efficiency with a fixed 1.23 kWh cost per charge.
I drove 498 miles this summer, and used 244.3 kWh of electricity to charge it during that period (including the per-charge overhead). At 7.5 cents per kWh (my cost in Fort Collins, CO), that means I spent $18.32 on electricity to drive, at an average cost of 3.7 cents per mile. At $4.00 per gallon gasoline, this is the equivalent to about 109 miles per gallon.