From: ask doggie
Almost any American plane which was used by other Nations had differences in armament, engines, and fuel tank sizes.
Many had a longer range because the American engines generally burnt with less tolerance, and were designed to burn more efficiently.
My favorite example of this was when my dad was building Allison engines in Indianapolis for the P 40 and P 39.
One day, Allison held a publicity stunt and blocked off west tenth street, where they then brought three P 40 models out, destined for the USAAF, China, and the RAF.
All three planes were given the same amount of fuel and started at the same time.
The Allison people bragged that the test would prove that the U.S. version was superior, and they were sure this display would show this.
Instead, the U.S. plane cut out first, then the RAF, and the Chinese was still chugging away.
The reason was that the planes were designed with different tolerances, and the importance of burning "lean" were made apparent (for those willing to learn).
That indeed is very cool, your father engineer for Allison ?.
Running lean in combustion engines is a way to increase fuel efficiency by trading longevity , the " tolerances " had to be changed due to running temperature increase. I heard many of the Allied nations request this due to their fuel shortages. Problem with this was at high RPM's they would run the risk of burning a hole in a piston/s ....