You aren't really using clutch surfaces to brake though, mostly you are relying on the friction and energy used to compress your intake air and blow it out the exhaust with either minimal or no combustion/fuel input, and if at speed air resistance. And everything else added up helps too like in friction in transmission and differentials and cam and crank bearings which are hopefully minimal, and also pumping coolant and oil and running the alternator, etc. In an automatic some will be lost lost in heating up fluid in the torque converter too.