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By contrast I've never seen the top end listed as anything other than 487 MPH in every book I own or have perused. I dunno.
So is sounds like the P-51H wasn't the high speed king it has been made out to be.
Both the P-47 M at 472mph and P-47 N at 467mph were faster and were operational in their respective theatres before the end of the war.
All I could find concerning actual speeds achieved was the following:
Figures are for aircraft in combat loaded condition. The high speed of the "D" - 435mph @ 32,000ft is estimated. The top speed of the "M" - 473mph @ 32,000ft and the "N" - 457mph @ 32,000ft are actual.