If "Car" is your seed keyword, you need to come up with some body keywords like:
red car
blue car
green car
petrol car
diesel car
At least 25 - 30 of those.
Then head over to keywordrevealer.com (it's a free tool, one of the best imo) and pop in your body keywords. You can do the same with google keyword planner. You will get about 800 keywords per body keyword, analyze them, find the ones you like check the google top 10 and there you go!
Long tails can be very powerful in bringing in traffic since there is less competition there. Also when you write an article, you will start ranking for some long tails that you did not even know about but used in your article naturally.