What happens if you mix your car gas?

If you mean mix it with another substance such as oil then it better be for a weed wacker! A typical car runs on straight 88 octane gasoline! Nothing else is suppose to go into the cars fuel tank unless its an approved additive for better mpg and/or other performance gains! If anything else is mixed into gasoline and poured into a cars engine the car will discontinue running! Or start sputtering, ticking, pinging etc!

Some older cars 70s back used leaded fuels and some now require a lead additive due to all street fuels being unleaded but that's for some classic cars only!