Think it only matters if you go from your current hummer to a next-gen prius? Think again. This article from grist explains how turning your gas-guzzler into something just even slightly more reasonable can have a huge impact.
“The Hummer H3 averages 15 MPG. Making an H3-equivalent that got just 18 mpg would be the fuel-saving equivalent of turning a Prius into a 100 mpg hypercar.”