When you give a cyclist a slice of bread that is 120 kCals, one would think that if the kCals per mile of the cyclist is 40 kCals per mile, the slice of bread would last 3 miles.
We all know this is ridiculous.
If it were true, a 300 km Audax rider would require 65 slices ( 5 loaves ) of bread to complete the event.
What happens in reality is that dependent upon the cyclists physique and the intensity of the riding, most of the required 40 kCals per mile are taken from fatty acids through lipolysis during the steady effort.
You will have to do your own experimentation, but I require about 4 kCals per mile on a <100 km ride, 12 kCals per mile on a 100><160 km ride and 20 kCals per mile on a >160 km ride.
On some flat 100 km rides, I don't bother eating anything extra above my daily BMR. An ice cream, maybe.
Leading to infinite mpg.