[JacobAziza]

I posted the same cost per mile analysis on the Mr Money Mustache board, and someone pointed out something important I had neglected in my numbers:

"I think you shouldn't be looking at the absolute calories burned when biking, but rather, the calorie differential between biking and whatever other activity the person would be doing, since we burn calories even when sleeping. From the same source, "Police, driving a squad car" burns 141 calories/hour (no mention if that includes donut-eating), so if you ride your bike instead of drive for that half hour, the calorie differential is only 180 calories, not 250 (this is good news for the people trying to save money by biking, but bad news for those trying to lose weight!) That brings the cost of the extra calories down to 27 cents.

Then, removing the "6 miles" from the numbers to simplify it, we arrive at 4.5 cents/mile for cycling fuel, and 17 cents/mile for driving costs."

Of course with that we are still assuming that the biking is an additional activity.
If we assume that a person is going to get some form of aerobic exercise regardless, and that they substituted biking to work for, say recreational running, then they aren't using any additional calories at all. Then, compared to the 17 cents a mile, it costs $520 a year to drive, vs $0 to bike which is reasonably substantial savings.