12th car is useless for a speed record attempt. It is too light, has too little suspension travel and the foam tyres will blow themselves apart before you reach anything like a decent speed.
For a record attempt you need something long, wide, heavy, good suspension travel and fitted with thick rubber tyres, preferably caps with a firm foam filling. That will also give you room for the sort of power LongRat suggests, and enough space for the sort of motor you will need. Air resistance increases as a square of the speed, so to go twice as fast you need four times the power! HTH
