I recently came over some numbers of GHGs released per energy in food produced, and the result is pretty flooring. I can see how animal-based food production is quite inefficient so let's ignore that for now and just look at plants.
For example rice releases 2 kg carbon = 7.6 kg CO2-equivalents per 1000 kcal. This would actually mean that it is more environmentally friendly to drive a car 10 km than to bike 10 km on a rice-based diet, because biking burns around 300 kcal = ~2.28 kg CO2e, while a normal compact car burns 0.5 liters of gasoline = ~1.9 kg CO2e. Are my numbers wrong? If not, how can this be?