I don't know about this stuff. Nancy Clark does recommend in her Sports Nutrition Guide Book that you have extra sodium before a long ride (or run, etc.) on a hot day. But her recommendations are for when you will be exercising for more than 4 hours in the heat.

I would not rely on water alone - for me a 40-mile ride usually takes 2.5-3 hours, so I would plan on having at least one bottle of gatorade, maybe two, in addition to several packets of gu, over the course of the ride.