What happens when the petrol dollar runs out of gas?

You'll have to push for another cheap means of transport......:)

A pun! You made a pun! That's wonderful! A good pun is its own reword.

We switch to the dog dollar.

The USA has been showing symptoms of collapse since shortly after WW2. Almost all nations eventually collapse if they don't get conquered first. (Only 21 countries have governments older than a century. Libya had a king for 2800 years, but switched to a president in 1975.) 

Timing and details of a collapse are unpredictable.