Cows vs. Cars: Greenhouse Gases

I was curious about the impact of one cow vs. one car in terms of greenhouse gases emitted. A cow doesn't emit carbon dioxide (well not that much) but rather methane gas as a byproduct of the bacteria that are digesting the grasses in its stomachs (yes a cow has 4 stomachs). So I tracked down some facts and figures:

A gallon of gasoline turns into 20 pounds of CO2 (source)
Average car drives 15,000 miles a year and gets 30 mpg (my estimates, might be a little high on the mpg estimate)
15,000 miles /30mpg = 500 gallons of gasoline a year * 20 pounds = 10,000 lbs of CO2 a year

A cow produces up to 90kg of methane a year (source .pdf)
Methane is over 20 times more effective in trapping heat in the atmosphere than carbon dioxide (source)
90 kg * 2.2 lb/kg = 200 lbs of methane * 20 = 4,000 lbs of CO2 equivalent greenhouse gases

A car emits 2.5 times as much greenhouse gases as a cow. Amazing that a cow emits that much and yet gets very little talk in terms of environment impact.

What about total world impact? There are 1.5 billion cows and buffalos (I am assuming that buffalos have similar methane outputs) (source from Eco-Economy using Amazon's search in book feature) and 532 million cars (same source different page). Using the 2.5 factor, cows emit 112% as much as the cars. This means cows have just as big an impact as cars and maybe even slightly more in terms of global warming. Maybe we should be focusing on making our cows more efficient rather than our cars.

What about 3rd world vs. 1st? In the land of the holy cow (that's India, not old school Gotham City) there are 313 million cows. In the land of the holy car (that's the US, as if I have to tell you) there are 200 million cars (source). US's cars emit 200 * 2.5/313 = 1.6 times as much as India's cows. The US still comes out on top in our polluting ways, but not by nearly as much as I would have assumed.

So then I was wondering, what if you could collect the methane from cows? What if instead of letting the methane go into the atmosphere where it causes harm, what if you capture it and use it as fuel, lessening our dependence on fossil fuels (methane is natural gas for those of you that didn't take o-chem)? I'm imagining a sci-fi like device with a tube that is inserted into one of the cow's stomachs that draws off the methane gas into a balloon on the side of the cow. Then when the cow goes into to get milked, you could siphon off the methane into a holding tank and then use it to heat your farm. Toss in a little generator and you could be producing electricity with it. Convert your car and you could be running your internal combustion engine on it. When I ran the numbers, it looks like methane sells for $0.172 /kg (source .pdf) * 90 kg = $15.50/yr in methane. Not a ton, but nothing to sneeze at either. Or as this document puts it:

This equates to about 120 litres of petrol which means a 200 cow herd would produce enough methane (24,000 litres petrol equivalents) per year to run an average sized car

And just when I thought that capturing the methane from the cows was an original idea, I run into this discussion board:

I've heard that cow flatuents (farts!) produce such an abundance of methane that it contributes to global warming. Has anyone heard or seen anyone trying to harness these cow farts? I have this funny image in my head of cows with tanks on their back with a tube going you know where. It would be an interesting way to make a living. Being a Cow Fart Tycoon.

Damn, I want to be a Cow Fart Tycoon.