> So go ahead and assume drivers eat. Bikers eat more.
Not that much more, and you're being disgustingly disingenuous by just grabbing the mid-point. To get 2000kcal, you're going to be eating more rice, potatoes and raw sugar i.e. carb-dense foods to fuel the ride. That's more like 2kg of CO2, so vastly below the 10kg of CO2.
I can safely exclude the beef, pork, etc. because that's food I'd eat "outside" of fuelling the ride. To spell it out for you: I won't eat more meat because I rode 65 miles, I'd eat more potatoes and cane sugar. Thus, that's what we measure in terms of excess CO2 produced vs just sitting on my couch.
Also, I'm being very generous to cars here. Most don't come close to achieving 65mpg, and certainly not on the route I measured with the steep climbs it involves.