The paper Diet, Energy and Global Warming compares the carbon footprint of plant and animal-based first to each other, and then to the carbon footprint of a Toyota Prius and Camry Solara, and Chevy Suburban. From the paper:
" The greenhouse gas emissions of various diets varies by as much as the difference between owning an average sedan versus a Sport Utility Vehicle under typical driving conditions."
" While for personal transportation the average American uses 1.7 × 107 – 6.8 × 107 BTU yr−1 , for food the average American uses roughly 4 × 107 BTU yr−1 . Thus there exists an order of magnitude parity in fossil energy consumption between dietary and personal transportation choices." The key number here is the 1.7 and 4 comparison since the exponent is the same.
Source: Gidon Eshel and Pamela Martin, Diet, Energy and Global Warming, Earth Interactions, May 2005, http://geosci.uchicago.edu/~gidon/papers/nutri/nutri3.pdf