[DennisAN]

I built up a cheap Harbor Freight emergency tool kit, but the only spare part I carry is the old (but in good shape) serpentine belt. What else could you possibly change by the side of the road? You can get any part shipped air express anywhere in the country overnight. If your car craps out you are going to be there a few days.

The most important thing to carry on long trips is a service manual(s). The mechanic in some tiny town needs it for reference.