I carry some car-specific parts but prefer to run an aggrressive preventative maintenance program rather than carry parts. Timing belt change done early precludes carrying timing belt (for instance - none of my cars has timing belt).
In some cases, like serpentine belt, I change it early and put the takeoff belt in the car parts storage as the spare.
I change belts, hoses and battery at 4 year interval just to be sure. Those haven't yet failed me except for the OEM battery on a new car failing at 3 years.
How do you keep all your goodies from melting into slag in the Arizona heat???