Atheists are not obliged to explain the origin or the diversity of life. If you don't feel like defending the Theory of Evolution just say "I don't know" and just because you think you do, doesn't mean you are right. There are many things we don't know and many things in the past, such as gravity, have been mysterious until a natural explanation was eventually found. Just because you can't explain something doesn't give the other person a license to fill the gap with a supernatural explanation. (Argument From Ignorance - a classic fallacy)
IMO it is usually not worth arguing Evolution with most believers, not on an individual leve. You will have to struggle against their mangled misconception of the theory, it's counter intuitive and defies "common sense" and you have to know quite a bit to be able to answer all the "facts" they will find on the Creationist sites.
Cars themselves do not reproduce. They are perhaps analogous to a seedless fruit. However, mechanical designs and aesthetic styles do show clear lineages and there is a selection process at work. Engineers and designers build on previous successes and abandon failures. A careful examination of the history of the automobile will likely show a marked pattern of experimentation, selection and inheritance but at a more abstract level than is found in living creatures.