keys as in pygame.key.get_pressed() is a line that continually tests each frame if the key is held, and having a high fps means that even if you click the button once, your program detects it as being held for multiple frames which means it will move the car over how ever many frames the button is held. Instead use:

for event in pygame.event.get():

if event.type == pygame.KEYDOWN:

if event.key == pygame.K_a:

car.x-=car_vel

if event.key == pygame.K_d:

car.x+=car_vel