John -
Here's one of several threads that explores the cost of gas versus electric. Detailed feedback starts at post #34:
Is range the only issue in gas vs. electric?
There are so many variables that affect mileage: If you're running at full-throttle most of the time (topping out at 19 mph.......right? :-) ), you'll get worse mileage than if you go 15-16 mph, and so on. Maybe you didn't have your tank full when you first started out and/or filled it up to the max when refueling. Probably best to compute an average mpg over several consecutive tanks to get a better read on it.
Hope that helps!
Bill
