According to the latest Dust to Dust report update, the Yaris is vastly superior to the Prius. http://www.cnwmr.com/nss-folder/automotiveenergy/107%2008%20... [XLS]

In energy dollars per mile, that report shows:

  Smart           $0.583
  Ion             $0.621
  Focus           $0.621
  Cavalier        $0.655
  Xb              $0.683
  Aveo            $0.693
  xA              $0.713
  Corolla         $0.748
  Fit             $0.799
  Yaris           $0.916
  Crown Victoria  $1.134
  Le Sabre        $1.462
  Lucerne         $1.589
  Park Avenue     $1.924
  Sprinter van    $2.173
  Prius           $2.191

Here's the page showing the collection of reports: http://www.cnwmr.com/nss-folder/automotiveenergy

This is the Prius-beating Dodge Sprinter van: http://images.google.com/images?q=sprinter+van

It's big. It weighs 4,784 lbs., empty. Fully loaded, it's 11,000 lbs. http://www.google.com/search?q=sprinter+van+weight