It's quite simple really, America out-produced everyone. Although these are from 1936 you can see how much America was ahead in motorised veichle production (Which gives the base for an impressive economy):
America 77.2%
Britain 7.8%
Germany 4.8%
France 3.5%
Canada 3.4%
Italy 0.9%
Others 2.4%
That's the percentage of motorised veichles built by them, in the world. Now, Japan isn't even in the top 6, so it could have never reached Americas production capability. Interestingly though, Russia isn't in the top six. I wonder where all their economy, designs, worker skill and tools came from? Britain, France, America and Germany perhaps?