Well, I don't think DT did the *wrong* thing, per-se. The maximum economical cruising speed for the Spit IX (from the manual) is 170mph, so they clearly went toward making those long missions more comfortable.
"The recommended speed for maximum range is 170 m.p.h. (147 kts) I.A.S. if the aircraft is lightly loaded. At heavy loads, especially if the rear fuselage tanks are full this speed can be increased to 200 m.p.h. (172 kts) I.A.S. without incurring a serious loss of range."
"On aircraft not fitted with interconnected throttle and propeller
controls
(a) With the supercharger switch at MS fly at the maximum
obtainable boost (not exceeding + 7 lb./sq.in.) and obtain the
recommended speed by reducing r.p.m, as
required.
NOTE.— (i) R.p.m should not be reduced below a minimum of 1,800. At low altitudes, therefore, it may be necessary to reduce boost or the recommended speed will be exceeded.
(ii) As the boost falls at high altitudes it will not be possible to maintain the recommended speed in low gear, even at maximum continuous r.p.m, and full throttle. It will then he necessary to set the supercharger switch to AUTO. Boost will thus be restored and it will be possible to reduce r.p.m, again (as outlined in(a) above).
(iii) In both low and high gears r.p.m, which promote
rough running should be avoided. "
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