Justice Rodney Hansen decided that grievous though Kinghorn's crime was, it did not meet the requirements of Section 104 of the Sentencing Act pertaining to maximum non-parole periods.
The Crown had argued Kinghorn killed Mrs McCullough in the course of another serious offence, forcing sex on her. But Justice Hansen said while Kinghorn's intentions may have been clear, the fact he did not actually touch the victim in a sexual way ruled out the use of that remedy.
He settled on 15 years as the minimum period of jail to be served, but discounted that by two years to recognise Kinghorn guilty plea, his remorse and other factors.
Earlier, the court heard victim impact reports from Mrs McCullough's family, including two sisters, 17-year-old son Ollie and husband Jeff.
Mrs McCullough was a devoted wife and mother, much loved by her family and friends, the judge said.
"Your actions have ripped the heart out of this family and left a void that will never be filled," he told Kinghorn.
"The reports speak for themselves. The words of the victims are the most eloquent testimony to the pain and the loss they have suffered.
"The terrible shock of her death and the abiding pain of her loss permeates every page of the reports."
Justice Hansen said Kinghorn's actions followed a night of drinking.
Around 1pm, he was driving on Frankley Rd when he saw Mrs McCullough walking on the grass verge.
He turned the car around and headed back towards her, at the last second swerving onto the verge at a speed between 27km/h and 36km/h, initially striking her on her legs.
He stopped the car, picked her up and placed her on the back seat.
Mrs McCullough was unconscious and suffered injuries to her head and brain, spine, chest wall and limbs. She died between three minutes and an hour after the impact, the judge said.
After driving his purple Holden Commodore along various country roads outside New Plymouth, Kinghorn eventually abandoned the vehicle to go to a nearby house to ask the occupant to phone police and his mother.
