(You can read all the sources and math at this site.)
Bicycling Wastes Gas?
Most people think that bicycling doesn't use gas, but actually it does. It takes lots of fossil fuel to produce the food for the cyclist's calories -- and cycling requires more food fuel than driving.
Of course, we can't just stop eating, but we can definitely choose what we eat, and here's the kicker: meat requires much more fossil fuel to produce than vegetables and grains. How much more? About 145 times more for beef than for potatoes.1 The reason for this is simple: Cattle consume 14 times more grain than they produce as meat. They're food factories in reverse. So it takes a lot more water, land, and of course, energy to produce that meat. In short, the more meat you eat, the more gas you waste.
David Pimentel of Cornell University calculates that it takes nearly twice as much fossil energy to produce a typical American diet than a pure vegetarian diet. This works out to about an extra 150 gallons of fossil fuels per year for a meat-eater. This means that meat-eaters are "driving" an extra eleven miles every day whether they really drive or not, when we look at how much extra fuel it takes to feed them.2
In fact, meat production is so wasteful that walking actually uses more fossil energy than driving, if the calories burned from walking come from a typical American diet:
"It is actually quite astounding how much energy is wasted by the standard American diet-style. Even driving many gas-guzzling luxury cars can conserve energy over walking -- that is, when the calories you burn walking come from the standard American diet! (62) This is because the energy needed to produce the food you would burn in walking a given distance is greater than the energy needed to fuel your car to travel the same distance, assuming that the car gets 24 miles per gallon or better."4
The same is not true of bicycling vs. driving, because bicycling is more than twice as efficient as walking (calories consumed per distance traveled) -- bicycling uses less fossil energy than driving even if the cyclist were eating nothing but beef.5 But to focus on this misses the point. It's no bombshell that cycling uses less fossil energy than driving. What's important is that meat-eaters use twice as much fossil energy as pure vegetarians -- whether they're bicycling or not.
What does this mean in practical terms?
It means that the amount of gas you use isn't just related to how you get from place to place, it's also related to what you eat. Meatless diets require twice as much fuel to produce than the standard American diet. Pimentel calculated that if the entire world ate the way the U.S. does, the planet's entire petroleum reserves would be exhausted in 13 years. The typical American could save almost as much gas by going vegetarian as by not driving.6
Food for thought.