Prior meals, heat, ride intensity, just the week you are having? All or any could play a part. Over the years, I know that I can ride just fine out to 40 to 60 miles with just my normal dinner, and a light breakfast.
On hot days at that distance I use water with crystal light. I have Gu in my jersey pocket 'just in case'. A the 60 and above range I know I have to pay more attention to my hydration and energy management. I will carbo load the night before, IO will use an energy drink during the ride. I know I will need to make a stop for both water and some energy replacement. If nothing else I take some snacks... but, I always take some cash for a meal if needed. I also ALWAYS have several Gu gels in my jersey pocket.
Find out your needs by noticing the differences your body tells you about. Hot vs cold rides, longer vs shorter... with breakfast and without. With energy drink and without. Once you get your arms around the variables, it can help to predict your needs and concenrs!