What happens if I put airplane fuel in my car?

There are two families. Light aviation with piston engines uses avgas (100LL), a high-octane leaded gasoline now being phased out. Jets and turboprops, which make up the vast majority, burn Jet A or Jet A-1, a refined kerosene with very strict requirements for freezing point (−47 °C for Jet A-1), water content, sulfur and thermal stability. Chemically, Jet A-1 is much closer to diesel than to gasoline: medium-chain hydrocarbons, low volatility, with a flash point above 38 °C.

A gasoline engine needs a fuel that evaporates easily and resists compression without detonating, which is what octane rating measures. Kerosene does the opposite: it evaporates poorly when cold, so starting will be difficult or impossible, and once warm it detonates easily, producing the knocking that can damage pistons and connecting rods. The mixture will burn incompletely, with smoke, power loss and carbon deposits on spark plugs, valves, the oxygen sensor and the catalytic converter. A mix mostly of gasoline with a bit of kerosene might go unnoticed for a few kilometers; a tank full of Jet A-1 won't.

Diesel engines ignite fuel by compression, and kerosene does this without difficulty: in fact, militaries have used a single kerosene-type fuel (JP-8, F-34) for decades for trucks, generators and aircraft alike. A modern diesel car will start and run on Jet A-1, but with problems: kerosene has less lubricity than diesel fuel, and the high-pressure pump and common-rail injectors, which are lubricated by the fuel itself, will wear out sooner; its density is lower, so power and fuel economy worsen; and the cetane number usually falls below the minimum, causing rougher starting. The aftertreatment systems (particulate filter, SCR catalyst) aren't calibrated for it either.

Jet fuel is taxed differently and often carries markers; using it on the road is tax fraud in most countries, and it isn't approved by any car manufacturer, which voids the warranty. In an emergency, a diesel can travel a few kilometers on kerosene with some two-stroke oil added to compensate for lubricity, as remote expeditions have done; a gasoline car cannot. The short answer is that it's not worth it: jet fuel isn't "better," just different, and it's designed for turbines that spin at constant temperature, not for a reciprocating engine.