Q: what is kafara of killing an animal such as cat in accident with car, vehicle (sent to site)
A: Accidents do take place. If you had killed a domesticated cat, merely compensate the owner. If you killed a stray cat, then in future be more cautious and alert when driving so this does not repeat itself.
Every creature of Allah (SWT) has value. Be remorseful when any creature is hurt without reason.
If possible, do any good action towards a cat with sincerity as a compensation, so you are more responsible when driving, this would also reduce some of your unneeded guilt.
Allah Certainly Knows Best.