Also, since it isn't "your" car, it is perfectly reasonable that on a longer taxi trip the cars just drive to a taxi depot when low on battery and have you switch cars or have the cars switch batteries (as demonstrated by Tesla).
I don't think any of the autopilot cars will be gas guzzlers. By the time they are allowed for consumer use as taxis, the Tesla 300 mile range battery will be half as expensive.
Just this weekend, me and my roomate went on a road trip to san diego and we rented a car because he didn't want to put the ~1200 miles on his car.