I suggest you read this article.
http://www.uwgb.edu/dutchs/pseudosc/200mpgcar.htm
Pretty much a myth. Under perfect conditions, -maybe- you could get 80 miles out of a gallon of gas. If there was no rolling resistance, friction or any other factor, maybe 400 miles coasting.
In other words, bringing facts and physics into this...it's impossible. Gas provides 130,000,000 joules of energy (approximately). That will only do so much work.