My Computer Dead Reckoning MK. 4A. by The London Name Plate Mfg. Co. Ltd.
using a cor. OAT of -56.5 and M15 gives 8560kn.
First time it's been used >M1.0 - poor thing looks quite worn out.
Correcting for radial change at 60000' gives 8535kn over the ground in still air.
Anyhoo, at M15 with a DT of about 9650C, speed would be the least of his problems.