Please note - I advocate both diet types, but it depends on the event.
As the above poster said - this is fuel, not nutrition. But selecting diesel or gas depends on what you want to do. The 24 hours of LeMons - diesel does well, F1 - not so much.
This is per OP 13-15 mph average (on a road bike on roads).
More links:
"
Fat is the muscles' primary fuel for low- to moderate-intensity exercise. Carbohydrate, which is the muscles' primary fuel for moderately high- to high-intensity exercise, is stored only in small amounts in the body. Consequently, carbohydrate fuel depletion is a major cause of fatigue during prolonged exercise at higher intensities, such as triathlons."
Dietary Fat and Endurance Athletes | ACTIVE
https://www.trainingpeaks.com/blog/f...ance-athletes/
This is different - it is fat adapting, but similar principles.
High-Fat Diets for Endurance | Runner's World