ok, there are 10 stages in which a dog goes through when they have pent up energy and in your case your pup expresses this through chasing cars!

here are some tips for you:
when you are out walking you pup, keep her interested in you by constantly talking to her, use a squeeky toy or her favourite toy to help as well!

when a car is coming by, you will notice her behaviour and posture changes and she will be going through the stages very quickly.
when you see the changes, tug on the lead to the side, dont pull her back. pulling the lead to the side keeps you in control and her in check.
do this every time, reward her with a tasty treat when she does what you want her to do and carry on with your walk. do this every time she looks at a car, goes to chase a car and when she is penting up her energy.

if you are consistant with this then she will stop doing this and walk with you and be more focused on the walk, not the cars!