Please rethink your statement. You cannot get more motive energy than the energy put into the propeller.

> I can make a car use 50% less fuel an infinite amount of times

No, you can't. Conservation of energy applies.

P.S. I didn't "add" them, either. 90% x 115% => 103.5%

That is not how it works.

1-((1-0.9)x0.85) = 0.915, 91.5%. 15% better.

(Your original point is not affected by this nitpicking, but:) A car, traveling any distance, between two points of constant height (in the gravity field), and neglecting work through friction (which is zero in the zero speed limit, albeit the movement is quite slow then, but a pumped out tunnel and mag lev gets you very close), requires no work, so no fuel. Technically.

> I can make a car use 50% less fuel an infinite amount of times,

At some point it'll be running on nuclear fission, as you won't be able to feed it a single whole atom. ;-)

Doesn't follow. One of the ways you can make a car use less fuel is by reducing drag and friction.

A propeller provides a certain thrust for a certain input power. If more power comes out than goes in, you can use it to construct a perpetual motion machine.

Your construction of what the number means doesn't make sense, and (therefore) you need to start over. It is generally a mistake to assume people quoting figures mean stupidly impossible things, even though sometimes they do.

Good luck with that.