When I am on a VCCA tour, I take a spare fuel pump and extra diaphragm, carburetor, points, condenser, distributor cap, rotor, spark plug wire(s), coil wire, length of electrical wire, spark plug(s), a few nuts, washers, screws, bolts, oil, grease, water pump lube, water or coolant mix, radiator hose, hose clamp, gasoline can (with gas) a small tool box, VOM, paper towel roll, hand cleaner, baby wipes, tow strap, gel pack w/light and air compressor (also powers hand held radio battery charger, cell phone charger and GPS), and my Gerber tool in the car. Probably take a few other parts that I am not remembering right now. In most cases I loan stuff to others when they need something. I also travel near the tail end so I can help as many as possible. I take a water pump, generator, starter, gaskets, Teflon tape, duct tape, rear axle, puller(s), seals, small floor jack, jack stands, many more tools, air tank, another gas can, more water, oil, etc. in my tow vehicle.
Over the 20+ years of VCCA touring, I have used or supplied nearly everything I take at one time or another. Luckily I (or my vehicles) haven't needed many parts or repairs "on the side of the road". Seems if I take a part I won't need it but often someone else does. As an example I changed out a carburetor on a '31 Coupe and then a few miles down the road changed the fuel pump. It still has my parts on it but I have rebuilt the other parts and returned them to the owner. Probably get my "used" parts back on the upcoming Southern Spring Tour.