Before all the snake oil salesmen started peddling 'sports drinks', Coke (with a tiny amount of salt) was THE drink, especially in hot weather. It's best to pour it out into a cup to kill most of the gas, but then it great.
I've done lot's of long distance riding and cycling events in the tropics using it. The best things is that it's available in most garages.
A UK university sports department did some comparative tests on elite athletes training hard in hot, humid weather and found they did equally well on Coke as on electrolyte drink. They also found that the caffeine allowed them to do better at the end of an event compared to plain sports drinks. That is why you see some gels now have 'caffeine added'.
For riders on things like PBP, don't be afraid of Coca Cola. If you lay off ALL caffeine for a month or six weeks before the ride, then the caffeine in coffee and in Coca Cola can be a useful tool in managing your tiredness. Colas also have the fluid quantity in it that a tiny expresso doesn't.