Here lets take a look at what
YOU are going to do....
Compare aircraft performance charts developed by warring powers during the 1940's.
How dumb is that??
Well lets see. All aircraft have position error corrections because your airspeed indicator does not measure actual ground speed and is subject to compressibility effects of the atmosphere.
Varying magnitudes of the errors described here are present within the pitot-static system of any aircraft Full details of a particular system must, therefore, be obtained from Aircraft Flight Manuals.
http://www.allstar.fiu.edu/aero/PSSI.htmHere are two position error corrections done by two different
Allied powers on the FW-190. None of the German insturmentation is different.
Same airspeed indicator, same pitot tube completely different corrections!!

And are completely different from Focke Wulfs own position correction.
Why?
Standard atmosphere was not standard! It was not standard in the United States, The United Kingdom, Germany, or anywhere in the world. The United States was actualling running three "standard" atmospheric models during the 1940's. Which one did Grumman use?
Every student of aerodynamics and flight mechanics is introduced to an atmosphere table, which allows one to determine the temperature, pressure, and density at any altitude. If you don't happen to have such a table handy, you may want to look at a sample atmosphere table in US units or SI units. The equations used are those adopted 15 October 1976 by the United States Committee on Extension to the Standard Atmosphere (COESA), representing 29 U.S. scientific and engineering organizations. The values selected in 1976 are slight modifications of those adopted in 1962. The equations and parameters used are documented in a book entitled U.S. Standard Atmosphere, 1976 published by the U.S. Government Printing Office, Washington, D.C. A summary of the definition of the 1976 atmosphere is on another page.
http://www.pdas.com/atmos.htmhttp://nis-www.lanl.gov/~stanleym/dissertation/node19.htmlAdd in the fact that aircraft performance is actually a percentage range of values:

It becomes a really silly endevour to compare aircraft that are less than 10 mph difference especially when that information comes from differing charts prepared in different countries.
All the best,
Crumpp