Ian Huntley thoroughly cleaned his car and replaced its tyres the day after the Soham girls disappeared, a court has heard.
Mr Huntley denies killing the girls
Richard Latham QC, prosecuting, told the Old Bailey in London that Mr Huntley's behaviour was that of a guilty man calmly "calculating his way forward".
The 29-year-old former school caretaker denies killing Holly Wells and Jessica Chapman, who disappeared from near their homes in the Cambridgeshire town on 4 August last year.
The prosecution says that Mr Huntley used his red Ford Fiesta car to dispose of Holly and Jessica's bodies, and that is why he was worried his tyre tread could be identified.
It also said the cleaning of the car the day after the girls disappeared and the replacement of the carpet lining the boot were significant.
Mr Latham said: "There was in progress on Monday a major attempt to sanitise the Fiesta."
The court was shown an invoice for a service of the car on 10 July 2002, 24 days before the girls disappeared.
The MOT showed the Centaur tyres had lost only between one and two millimetres of tread since being fitted the year before, the prosecution maintained.
The tyres being hardly worn, Mr Latham said there had been no need to change them.
But before Mr Huntley's arrest, his car was towed to a garage for forensic testing and was found to have been fitted with four brand new tyres, the jury heard.
Mr Latham said: "If you've got a guilty conscience you may be prepared to spend a large sum of money to throw away a perfectly good set of tyres."
Mr Latham said mobile phone records showed Mr Huntley had been in the nearby town of Ely on Monday afternoon, where an invoice at a tyre depot indicated four new Sava Effecta tyres had been purchased.
"The man who turned up asked for a different registration number to be put on the paperwork and slipped £10 to the mechanic in order for that to be achieved," said Mr Latham.
"More particularly, one of the mechanics noticed the tyres coming off were in surprisingly good condition, with four to six millimetres of tread depth."
Mr Latham said the person who would pursue this course of action was "the person who went there with the bodies".
Mr Latham said a search of documentation and receipts relating to the car in Mr Huntley's home had not revealed the receipt for the new tyres, because he had wanted to get rid of the evidence.
The prosecution alleged Mr Huntley was cleaning his car while locals searched for Holly and Jessica on the Monday evening, which was perceived as being "insensitive".
The prosecution suggested there was a more "sinister" motive.
As well as the cleaning of the car, when police later searched it, they found a piece of domestic carpet "cut to make a rough fit" had replaced the factory-fitted carpet in the boot, the court heard.
The next day Mr Huntley drove the car to Grimsby to collect Ms Carr.
A neighbour of Ms Carr's mother saw the pair standing near the car, the prosecution said.
Describing what the neighbour saw, Mr Latham said: "The car boot was open and both of them appeared to be looking into it.
"Maxine was sobbing and Ian just looked really thin and pale.
"As she opened her gate it made a noise and they both looked round and at her.
"Ian closed the boot. Maxine put her head down and continued to cry."
Mr Latham added: "Members of the jury, remember this - the appearance of the Ford Fiesta had been transformed and we are going to suggest there was a significant transformation in the boot.
"What had been in the boot which had suddenly necessitated a new carpet?"
Ms Carr, 26, has denied two charges of assisting an offender and one of conspiring to pervert the course of justice.
Mr Huntley denies two counts of murder but has admitted a single charge of conspiring to pervert the course of justice.