okay, theres one more scenario. I noticed it upon reread.

If you rent a car today at noon and will tell them that you will bring it back at 230 tomorrow, then there is no grace period. Your rental quote used the grace period from noon to 12:59 and then started charging hourly. In that scenario though, if you bring back the car early, it will cost less. If you bring it back late, (15:00) then it will charge you for one more hour, or the daily rate, whichever is cheaper. So in that scenario, there really isnt a grace period because the grace period was used in the quote itself. Once the rental overage ( any amount over the 24 hour day plus 59 minutes grace) exceeds three hours, it is usually (more often than not) cheaper to take the daily rate and the computer automatically chooses that rate.

thats probably the most confusing of the scenarios.