Every country is entitled to have a conspiracy theory, even such a powerful country as the United States. Obviously less powerful countries are normally more prone to various conspiracy theories either because the fact of their weakness makes them believe, more powerful states are taking advantage of them or simply because they have underdeveloped media, that is substututed by word of mouth.
At last I have located a conspiracy theory in the US as well. Of course, it is about oil (and may not be far away from the truth, mind you, at least in oblique ways).
So a little bird has told me, that big American companies in 1970s following the energy crisis built electric cars and distributed them among some families for testing purposes. After years of testing, the electric cars proved to be very handy indeed, so that their testers (the families) did not want to give them back. However, they were taken from them as they belonged to the big automobile companies and were subsequently destroyed (in cold blood, I would add for more horror).
The conclusion is that big automobile companies do not want to produce fuel efficient or alternative fuel automobiles. They have the technology, but they realise that it is going to be too unprofitable for them and for oil companies as well.
If this conspiracy theory is right, then this post should receive no references and die in oblivion:)