The driver of the vehicle, Henry Long, received a 16-year sentence and is not eligible for early release.
Speaking on the Today programme, Thames Valley Police chief Jason Hogg urged the government to reconsider its position.
Only prisoners convicted of murder, rape, grooming, and serious child sex offences will be excluded from the scheme in England and Wales – while those convicted of manslaughter remain eligible.
“I fully understand the challenges around the crisis in the prison system,” he said.
“But the decision made and where that line has been drawn, I do not believe that people responsible for taking the life of another citizen in our country should be released early.”
He said: “I was informed yesterday that the Police Federation are considering legal action, I am in discussions with the Police Federation with regards to this matter”.
Asked whether this meant he would join them, he said: “I think we need to get some legal advice in relation to this. This is something I certainly would consider.”
Following PC Harper’s death and a campaign by his widow Lissie, Parliament passed “Harper’s Law”, which gives mandatory life sentences to the killers of emergency services workers. It also doubles the maximum penalty from 12 months to two years for those who assault police or other emergency workers.
But the new rules – introduced in 2022 – did not apply retrospectively to the sentencing of PC Harper’s killers, and so do not prevent them from being released early.
Hogg said: “The law isn’t retrospective and we all understand that, but the fact that they are released early simply seems perverse.”
Aileen O’Connor, branch chairman of Thames Valley Police Federation told BBC Breakfast her organisation was exploring whether legal action was an option.
“If it is an avenue we can explore then I’ll get behind that 110%,” she said.
As for what shape a potential legal challenge might take, she said: “Ultimately that’s for the specialists, the barristers and those in that legal sector to make that decision whether there is a challenge there.
“If there is a challenge then we will look to take that challenge.”
Sentencing minister Jake Richards told the BBC it would be wrong to pick out individual cases and block two of PC Harper’s killers from early release.
Although the government had reviewed what sentences would be excluded from the early release scheme, he confirmed “manslaughter is not one of those offences”.
On Wednesday the Ministry of Justice released a new list of exclusions that will be exempt from the early release scheme.
It said rapists, as well as criminals sentenced for child sex offences and grooming and conspiracy offences for these crimes, will no longer be eligible to leave prison early.
Murderers and terrorists were already not eligible for the early release scheme.
Richards said: “It would be wrong to pick out individual cases within different offending cohorts. That wouldn’t be right. It wouldn’t be lawful either. So it is correct that those two individuals will be subject to the new regime from October”.
