The average American consumes 38.87 gallons of soda year. 43% of Coke sales are Diet, I extrapolated that across the market, since Coke has a pretty dominant ma

The average American consumes 38.87 gallons of soda year. 43% of Coke sales are Diet, I extrapolated that across the market, since Coke has a pretty dominant market share anyway. That's 16.7 gallons of diet soda per American, or 178 cans of soda. Multiply that by 150 calories/can, divide by 365, you get the 73 calories. As I mentioned before, that's a side effect of averaging across all Americans, a fraction of who don't drink soda, and a (much larger, in both senses of the word) who drink sugared soda. The population that those calories would realistically be distributed over, the diet soda drinkers, is somewhere between 20% and 40% of the population, hence the "multiply by 2-5".