The Mustang E' was actually built to compete in a fuel saving competition held annually from 1930's to 1968 called Mobilgas Economy Run.    Every automotive manufacturer there was competed in the competition.  Professional drivers were used, so many Indy Car drivers were used that the competition was held in April so they could be free to practice and run the Indy 500 race in May.  It was a cross country competition from Los Angeles to New York using a different route every year.  This was governed by USAC.  So Ford had a new engine for the 69' model and used the Mustang in the 68' competition to show it off.  
They used the next years vehicles in the competition to promote the upcoming year of vehicles.   The Ford Mustang engineers went overboard on these cars trying to make them as competitive as possible. The economy run was cancelled in 1968 so the 1969 Mustang E never run sadly‚Ķ.

The Mustang E was aimed at the Economy minded customer. This newspaper article was published in the November 17, 1968 edition of the Independent Press Telegram from Long Beach, California. The article takes many quotes from John Naughton, Ford's vice president and general manager. The article is very similar to those published in many other newspapers around the same time. John Naughton is quoted, "The new Mustang E” will meet the limited, but particular, requirements of customers who want sport car appearance and handling combined with outstanding economy. This new Mustang will provide up to 15 per cent greater fuel economy than Mustangs equipped with similar drivetrain components." It states that the economy is mostly due to the 2.33 rear end and larger than stock torque converter.