The N.Y. Times article "The World : Rethinking the Meat-Guzzler" explains:
"To put the energy-using demand of meat production into easy-to-understand terms, Gidon Eshel, a geophysicist at the Bard Center, and Pamela A. Martin, an assistant professor of geophysics at the University of Chicago, calculated that if Americans were to reduce meat consumption by just 20 percent it would be as if we all switched from a standard sedan — a Camry, say — to the ultra-efficient Prius."
Reducing beef consumption also reduces methane emissions a lot! Methane is twenty times more potent than carbon dioxide as a greenhouse gas and represented 28% of US methane emissisions. Methane production occurs in the rumen as bacteria break down roughage. This methane is then eructated by belching.
Now I just need to make this into a clever bumper sticker!