I'm not sure there is one that will grow with him. We've got the Clix Car Safe for Ted (
http://www.companyofanimals.co.uk/products/safety/travel/clix-carsafe), although he will need a bigger size as he grows. It is made out of a continuous loop of seat belt webbing, so should be strong in the event of an accident. Personally I wouldn't use a normal walking harness in the car, as they won't have been designed to withstand that type of impact.

The Clix can be used as a walking harness when you get to your destination.
I don't actually use the harness that much, as we have found Ted settles better in a crate, probably because he can't see us. He did have a tendency to try and lunge across the back seat and nibble my son when in the harness. I probably won't get another one until he's all grown up and sensible.

Enjoy your outings with pupster, you'll have such fun!
