Driver Jacob Hocking, 22, Jackson Edwards, 20, and Adam Richards, 21, all from Corwen, were killed in the A5 collision on 21 November last year.
Anthony Steel, 74, from Birkenhead, who was in the other car, also died.
North Wales east and central coroner John Gittins concluded a road traffic collision caused their deaths.
The Ruthin inquest heard the collision involved a Vauxhall Astra the young men were travelling in and a Vauxhall Zafira being driven by Mr Steel.
Witness Brian Mcmulkin said he was driving towards Conwy when he saw the Astra driven by Mr Hocking come towards him around a bend at speed.
He told the hearing he was struck by the "speed of it" and the driver was "fighting with the wheel a little bit".
Mr Mcmulkin said he looked in his rear view mirror and saw the car's "back end" before telling his wife: "He's lost it," but did not see the collision.
A North Wales Police forensic vehicle examiner told the inquest the Astra's rear near-side tyre was found to have a pressure reading of 13 psi - 54% below the standard recommended.
He said it may have "introduced an element of over-steer".
The inquest heard there was no police evidence to suggest Mr Hocking was speeding.
Mr Gittins said he was satisfied the under-inflated tyre was a "significant contributory factor".
The deaths were a "reminder to us all about the importance of vehicle maintenance," he added.
A two-car crash in Denbighshire which killed four people was likely to have been caused by an under-inflated tyre, an inquest has heard.