Looking at P40 Speed Performance Numbers...

[JG27_PapaFly]

Originally Posted by ElAurens

Seeing that the P40s were getting ever porkier Curtiss made an effort to lighten the last production model, the N, and they were successful, getting it's BASIC weight down to 6717lbs, from the portly M's 6899lbs. However, it's top speed is listed as some 15 to 20mph lower than the P 40E, at 345 ish mph. at a slightly higher altitude. Still at MILITARY, not COMBAT power.

Something is not adding up here, as other sources I recall show the Ns as the fastest production models.

I'll keep looking for answers, but anyone else have any clues?

Depending on a plane's specific configuration, a decrease in wingload can increase drag in high-speed flight. Sailplanes get significantly improved lift-to-drag ratios and therefore higher cross-country speeds at high speeds by filling water into their wing tanks.

One other contributing aspect is that a change in wingloading often changes the relative positons of the center of gravity and center of lift. This influences the trim drag. CG position has to be optimized in order to minimize trim drag at high speeds. However, optimal CG placement for high-speed flight might be prohibitive due to stability/safety reasons.