You really shouldn't. We understand why it might be appealing to use your dog's walking harness to strap them in – after all, these things usually include buckles to attach to the car and will save you a chunk of money. However, dog car harnesses are designed for a) restraining your dog so they're in a safe position if a crash occurs, and b) ensuring your dog has the correct padding to protect delicate areas of their body (most notably, their chest and neck) in the event of a collision.