The cruciate injury to his back left leg happened a few months ago, and the arthritis got worse in the six months before that.
Knill said the vet felt Jed was too old for surgery but was "not ready to go yet".
"He's still a healthy dog otherwise," Knill said.
She bought the cart online when she saw Jed deteriorating with arthritis, and he's been riding in it since the end of last year. She takes him for walks in Whangārei around the Hātea Loop and various spots in the Far North.
"It gets him out, stimulates his brain. He chats to other dogs, other people talk to him and he has a life."
He can walk a maximum of 250m to 300m at a time. The cart has sides which fold down to make it easier for Jed to get in and out.
"Pulling him actually is a really good workout for me."
That's because he weighs more than 30kg.
Knill takes Jed out in the cart every day.
"He loves it."
And they get to socialise. Whenever they go out "every second person" stops and talks to them.
Knill had a smaller border collie before Jed who also got arthritis when she got old. Knill pushed her around in a pushchair in her final weeks, and that's where the idea came from for Jed.
She doesn't know how long Jed has left.
"Take it day by day and try to give him a life, that's all I can do."
