And to complicate a simple question further...
From flight reports from Boscombe Down:
The Blenheim Mk 1, with standard engines, had a 'true' top speed of 274 mph @ 13,000ft. This was using Rotol (standard) propellers (2750rpm +5psi).
With careful cleaning, smoothing (including wing fillets) and engine boosted from +5psi to +9psi, speed increased to 296mph (true).
The Blenheim Mk IV, demonstrated a maximum speed of 255 mph @ 2,000ft. Flight testing also revealed different speeds depending on the manufacturer. A Blenheim Mk IV produced by Avro, for example proved to be 5 mph faster than the original Bristol-made aircraft.
The report goes on to mention that better speeds were expected utilising a special paint, called Camotint.
Does anyone know anything about Camotint?