Here is a good article by the hammer guys-download their endurance athletes nutrition guide, its a good read on fueling, I don't believe everything they say but they have some good info.

After racing a few years I can slam any type of gel/carb product with no issues. I do a gel every 20-30 min during a race-if over 2 hours I will hit a clif bar. Approx. 240 calories per hour works for me.

The Top 10 - The biggest mistakes endurance athletes make | Hammer Nutrition