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 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 on Wednesday: “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.”
After campaigning by PC Harper’s wife Lissie following his death, parliament passed “Harper’s Law”, which gives mandatory life sentences to the killers of emergency services workers. It 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 in the sentencing of the police officer’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.”
Justice Minister Jake Richards told the 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”.
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”.
On Tuesday Justice Secretary Alex Norris warned a prison capacity crisis would be likely if fewer prisoners were allowed early release.
Norris apologised to families who would be affected by the early release scheme and stressed that the government wanted to go further “but we cannot”.
“We have to make this decision, we’ve been able to make some changes to it, but we have to have the rest of the sentencing act changes otherwise we run out of prison places in November and that can’t happen,” he said.
The justice secretary said 5,000 extra releases will take place in 10 batches from October.
Approximately 1,000 prisoners will now not be released early following the revision to the scheme, he clarified.
The government also announced an extra £10m will go to front-line victim support services, including a new national helpline.