Always-carry list, specific to roadside 'aw crap' situations:
TIRES
--At least one hydraulic jack plus the factory jack and its handle
--Breaker bar with the proper socket
--Bigass screwdriver for pulling the center cap off
--Can of Free-All for the lug nuts. (That stuff is INSANE for dissolving crap in the threads! I've only ever found it at O'Reilly, but it's worth having a can in the tool box.)
--TWO spares. Because if you have a problem, you then do not have a spare.
UNDER-HOOD
--Belt. Usually the most-recent takeoff. I replace the thing every couple of years whether it looks bad or not. Take a pic of and PRINT IT OUT, or draw a diagram of, the belt routing and stick it either in with the belt or in the glove box....or make two and put 'em in both places!
--Whatever it takes to twist the tensioner. In my case, the breaker bar for the lug nuts will fit in the molded hole on the tensioner.
--A package or two of that hose wrap tape. May be worthless and I've never had to use it, but it doesn't take up much space.
--The most-recent takeoffs of the big hoses, and a foot or two of heater hose to loop if a heater hose busts.
--Spare clamps for whatever sizes needed.
--Coil pack and boot, and the 8mm socket to replace it. One goes bad and it gets noticeable, but like social crying in a baby nursery, if one goes bad it'll give another one the idea and if you lose two you ain't getting up some of these hills around here and on flat ground it'll shake like a dog shitting a sponge. V8's dont like running on six.
MISC (besides the fluids, not much bigger than a shoebox worth of stuff)
--Fuse assortment
--Roll of electrical tape
--Duct tape
--Paracord
--Bailing wire
--Taillight bulb and headlight bulb (rest of the bulbs are nice but those keep ya from hitting or getting hit
--Enough water to fill an empty cooling system, my case that's a little over 2 gallons.
--Make-up engine oil and trans fluid (trans fluid can usually go into power steering pump as well, but motor oil cannot.)
--Pint of brake fluid
--Jug of bug juice. Yeah, 99% of the time that's just a space waster. BUT.... one time I hit a buzzard and splattered his innards all over the windshield. Blew out the entire reservoir trying to clean it, and the window was still greasy and smeared. Would've been a MESS driving into the setting sun if I'd have had to use plain water or tried to go on as-is.
--Roll of paper towels and hand cleaner