I am not sure this is the case everywhere but it is in Chicago where I live (and probably true other places too so worth checking on).
I went to rent a Zipcar for a day (24 hours). The website told me the rate for 24 hours was $107 for this particular car.
I plugged in a 24h time (6a to 6a the following day) and the total was $131 and change.
So I plugged in 6a to 5:30a the following day and the rate was $98.
That is a substantial difference. So much so I repeated the test numerous times with other cars and got similar results.
Still thinking something was amiss I called Zipcar and described what was happening. At first the agent was dubious but she plugged in the same times I had as described above and got the same result. She was so puzzled at that she put me on hold to talk to a supervisor.
The answer is it is a Chicago city tax which, apparently, kicks in for a 24 hour rental but but not for a 23.5 hour rental.
So, if you rent Zipcars be aware of this. Don’t take my word for it, try it out for yourself.
The difference in price is substantial and in most cases if you are renting for 24 hours returning the car 30 minutes early is likely not a problem.