The most common issue that you might be able to fix roadside is a tire puncture.
In each vehicle, there's at least:
- A
Nealey tire repair kit (basically the finest sticky strings known to man; they work a bit differently, don't need glue, and they're amazing),
- Some sort of multitool with pliers
- An air compressor (the high volume model from HF is quite nice, about $30-$35; on motorcycles, I carry a "Slime" brand compact compressir). And a tire pressure gauge.
- A flashlight, probably two or more
- Maybe something to throw on the ground while wrestling a tire and wheel.
- On motorcycles, the means to elevate and remove the wheels should that prove necessary.
And of course in cars, you have the stock spare, lug wrench, and jack. Best to test the lug wrench to make sure it actually will do the job, and consider tucking in something easier to use. There are also some really weird, cheap little cigarette lighter powered electric impact wrenches out there that will actually do a pretty good job.
A jump pack, assuming it's charged, is also good to have around. Or perhaps jumper cables. Almost always used to help someone else; I replace car batteries before they die.
Of course, I often carry lots of other stuff, too. Volt meter is handy, and a few spare fuses are always tucked into the glovebox. Paper towels and/or hand wipes are nice to have.