the EPA can certify an MPG rating based on someone else's date the same way the FDA certifies new drugs based on the manufacturers data.  Because the manufacturers said so.  The same way US currency with 100 on it buys only 50 worth of goods.  The govt (manufacturer) says it's 100.

as long as capitalism exists there will be sub-standard factories in some 3rd world country, which at some point may be the USA.  if the unions get into Walmart some will think thisnis great but the same sub-standard factories will make the products while union members get paid more to put them on the shelf.