A lot of this comes down to what you practice. All the studies in the world can’t help figure out what’s right for you, or what will make it out of the bottle cage or jersey pocket and into your mouth.
A great starting point is this blog post from Asker Jeukendrup, which summarizes current broad recommendations:
http://www.mysportscience.com/single-post/2015/05/27/Recommendations-for-carb-intake-during-exercise
A trickier problem is figuring out the composition of various drinks, chews, gels … etc. Some educated guesses are necessary to convert generic ingredients into glucose vs. fructose vs. other types of carbs.
I have used Skratch and Honey Stinger chews with good results, targeting about 70-80 grams of carbs per hour. I also tried Science in Sport BetaFuel, which I liked but found that I need to also take in fluid (without any mix) since 500 mL/hour left me feeling underhydrated.