You don't want to fill your trunk with spares and tools, but you need a few things just in case. Prepare for the most common, easily fixed situations and pack accordingly.
- Tool kit. I have a flat folding tool kit I got from Lowe's that includes sockets and wrenches and a multi-tip screwdriver, to which I added a razor knife and a set of allen wrenches. 10x20x2 inches like a big book, so it slides into the trunk just about anywhere without taking up much room. Much better than a tool box.
- Fan belt(s). Common failures and they don't take up much space.
- Cap/rotor/points/condenser/coil/plugs. Again, small things that don't take up much room but are common issues on the road.
- Quart of oil, gallon of coolant
- Fuel line and spare hose clamps. Just a few feet, just in case.
- Spare electric fuel pump. If you already have one on the car in addition to your mechanical pump/vacuum tank, you probably won't need it. If you don't, you might need it if your primary system goes south. Easy to install by the side of the road to get you home in a pinch. The Airtex pumps don't take up much room and are simple to install almost anywhere.
- Jack and lug wrench. I have a small aluminum jack that works well and clears all our cars' frames. It doesn't weigh much, although it is oddly shaped and takes up a bit of space. Recently had a flat and we had a spare on board but no jack and had to call for assistance. Ugh.
- One of those small electrical kits from the auto parts store. A few feet of wire, various connectors, and a set of electrical pliers. If you feel like you'll be doing some serious electrical troubleshooting on the side of the road, include a test light.
That list of stuff shouldn't take up much room in your trunk, beyond the jack and gallon of coolant. If you need more than that, you're calling a flatbed anyway. Prepare for the common things to go wrong, be ready to tackle issues as they arise, and don't fill the trunk with junk you'll never use. If your car is properly sorted and you've been driving it regularly, these things should all be placebos anyway. I take them mostly for good luck, but have never needed to repair anything by the side of the road beyond replacing a voltage regulator in a hotel parking lot, which required only a screwdriver.
Drive your car a lot. Fix whatever goes wrong. That's by far the best way to make it reliable for trips.
