If you maintain the cars well, eventually the roadside tool kit will get smaller and smaller, because you know you've addressed the critical maintenance items one by one.

I used to carry my tool box in my trunk, with all my basic tools (it's not much) and a few filters, belts, jumper cables, etc. Now I leave the toolbox behind but carry the multi-tool, a set of main and pre-filters, fuel return lines I've never needed to use, spare lug bolts, and the jumper cables. Funny thing is I've never had to use them at all, but may prove useful if I need them for somebody else, which has happened a few times when I needed to boost their batteries or lend them an adjustable wrench for clogged filters.

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Tools I would have:
Quality full set of open/box end wrenches.
Quality full set of 1/4 and 3/8 Metric Sockets.
Hammer for those hard to shift things.
Quality set of screwdrivers both large and small regular and phillips. MAGNETIZED.
Needlenose pliers.
Adjustable wrenchs large and small.
Channel locks instead of pliers.
Razor knife.
Vice Grips.
Small shop jack.
Air pump/light.
Wheel chock of some kind.

Parts onhand:
Belts
Filters
Fuses
Spare wire.
Spare hoses.
Duct Tape!
Oil, Trans Fluid, Brake Fluid, PS Fluid, Hand Cleaner and Towels.
Old but tested good glowplugs, injectors w/sealing rings.
Throttle Bushing for firewall.
Bulb assortment.

This is why my 300D tends to squat.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)