It is alleged Richardson stopped at the country lane and twice got out of the vehicle in attempts to pull Mr Mullins free, which he also denies.
Richardson then mounted the grass verge in his Volvo and finally dislodged him before he reversed over Mr Mullins causing “crushing” injuries to his chest.
The defendant’s cousin, front-seat passenger Gabrielle Debotte, previously told the court that both heard “moaning” shortly after the collision in Paddock Lane and that he went on to say: “He is under the car.”
But Mr Hamilton said Ms Debotte had “changed” her accounts of the journey in various statements to the police and in evidence to the jury.
He said: “One of the things that you will have to do in your assessment of Gabrielle Debotte is to ask yourself, is she a honest, accurate and reliable witness?
“We say she is a narrator prepared to say whatever it takes to remove her further away from any sort of responsibility, and as a consequence to shift Simon Richardson ever closer to whatever suits her at the moment.”
He added the prosecution had been “blind to the distinct possibility” that Mr Mullins was run over by one of the front wheels of the car in the initial moments of the earlier collision.
Jurors have been told that to convict Richardson of murder they must be sure that for at least part of the time from when Mr Mullins was dragged under the car to being left on the grass verge that he intended to kill or seriously harm him.
Trial judge Mrs Justice Brunner said they must also be certain the injuries suffered during that period were more than a minimal contribution to Mr Mullins’s death.
On the alternative offence of manslaughter, she said jurors must be sure that the defendant assaulted Mr Mullins which, again, was more than a minimal contribution to his death.
The defence argued there was no assault committed because Richardson did not realise that Mr Mullins was or possibly might have been under the car.
Richardson denies murder, manslaughter and a separate allegation of dangerous driving on 21 April following a police pursuit before he was arrested.