ummm......when you say extend their life what do you mean by that? What kind of fish do you have, how big is the tank, and if its in your car i am assuming it does not have a filter nor a light nor an air pump?
keeping fish in ur car isnt a good idea. the conditions r to unstable. in winter it would get extremly cold in the car and in summer it would get extremly hot in the car. even in the winter it could get very hot in the car if the sun is out. depends r were u live tho. u might be able to if u live in a cold place and keep coldwater fish.