[Murph]

Lordish, are you familiar with the difference between "True Air Speed" (TAS) and "Indicated Air Speed" (IAS)? In short, it means that the higher altitude a plane flies at, The LESS accurate the airspeed indicator reading is. This is because of the thinner air at altitude.
So your actual or TAS might be quite higher than the indicator says it is. (Also trim is really critical with the P51- but that's another subject).