the problem is usually you have the fire extinguisher in the trunk. so by the time you get to it, you better just run away from the car.
most car fires are large enough that the small auto extinguisher we all buy will not put out a fire they are basically just to give you time to jump in and rescue somebody or blow the fire out of your path of escape. you would need a Large extinguisher like hung in a building and not many of us are going to keep that inside the car cabin so it goes in the trunk. you should carry a small extinguisher but you you have to mount it somewhere inside the car where you can get to it. that presents a problem because you drill holes and mount it inside the car and it then you have to look at it all the time. the really tiny ones that fit in the glove box are useless.
another issue is you have to remember to inspect them every 6 months because they leak slow and need to be recharged every so often or you have to buy a replacement.
most of the time the fire extinguisher in your car is used to help somebody else in another car that is on fire.
I kept an extinguisher in my trunk with the thought not to save my car but somebody else. it cost about 40 bucks but again you have to keep replacing it like every 1-2 years.
years ago i was welding my floor pan on the passenger side of another car and i caught the undercoating on fire and i didn't know i had right away so the fire was going for about a minute under the car, i unloaded 2 huge wall hung fire extinguishers under the car, called 911 was firing a garden hose under the car until the fire department came it was still burning when they showed up. at that point i didn't care about the car just my house as the car was in the garage. about another minute and i would of ran away from the house.
the whole thing happened in about 2-3 minutes and the cops and fire dept took another 5 to come.
was lucky i had the hose turned on for the water since i washed another car with it about an hour before.
so just remember carry an extinguisher but realize it will most likely help somebody else and not you.
for small engine fires just remember that powder that comes out to put the flames out gets EVERYWHERE and makes a huge mess, if the engine was running with when you put it out it will get inside also.
some car shows have rules about requiring each car attending to have a fire extinguisher in the trunk or cabin, most have banned those plexy or glass fuel 2 or 3 piece filers as they are known fire hazards. no crazy exhaust mods like spark plugs in the tail pipes for flames either.
if you ever went to a track day the race officials will go all over your car and ok you for getting on the track there are a bunch of things they don't allow like very long rubber fuel hoses, some braided lines are not allowed, again certain filters or fuel rails are considered dangerous.
oh ethanol: I've have seen it eat old fuel lines. I have also seen it soften rubber fuel hoses to the point they weep fuel.
i had to replace the soft fuel lines on my car when NY changed from Gas to 10% ethanol.
every couple of weeks because I'm paranoid i take a look at the fuel lines in the front and back of the car just to make sure they are ok and i check for any pooled gas on the intake,,, all that because it happened to me once, i walked into the garage it stunk of gas i opened the hood and freaked out.
i had to change the hoses to ethanol rated ones. and swap the black rubber holly seals for the new ethanol green rubber seals.