The AEC Regal T (coach) is not powerful enough to reach its maximum speed. It has a maximum speed of 84km/h but only enough power for 81km/h empty and 64km/h full.
I am not sure if this an intentional lack of power or accidental.
The power would have been based on real life statistics - the maximum speed probably guessed, as maximum speed for 'buses in the UK was generally set by the government and was revised upwards a number of time during the possible useful life of these vehicles.
Thank you for pointing this out, however.