Originally Posted By: thseng
In my experience, the temperature in a vehicle seems to drop very quickly as soon as you turn off the heater.

Perhaps you did not close all the vents, so the hot air just escaped? Also it's a typical mistake (at least here in California) when people are trying to use car heater or cooler leaving the main ventilation switch on intake (instead of recirculating)... And of course the heat retention depends on a particular vehicle design.