In 1908 the best selling vehicle in American was the Ford Model T. It got 17 mpg.
In 2007 the best selling vehicle was the Ford F-150. It gets 16 mpg.
http://www.dailyfueleconomytip.com/miscellaneous/100-years-of-improvement/ And reason number 1 not to wander off into the Everglades...
http://news.bbc.co.uk/2/hi/americas/4313978.stm