As many as 13% of drivers sit in the wrong position for their car seatbelt to be effective in a head-on crash, a survey showed.
These motorists were sitting too far back and were in danger of "submarining", where the occupant slips under the belt in a crash, said the British Osteopathic Association (BOA), which conducted the poll.
The BOA said, that to be effective, the belt should be sitting over the bones of the pelvis and not the stomach - preventing internal injuries - and in contact with the shoulder to prevent serious neck injury.
The poll of 1,435 UK adults also showed many drivers do not position themselves close enough to the head restraint, increasing the risk of serious whiplash injuries.
Also, only 6% regularly adjust the head restraint and 51% never do so.
Published to coincide with the start of Back Care Awareness Week, the survey also showed 14% of drivers sit too close to the steering wheel, risking a serious chest injury from the airbag in an accident.
The poll revealed that 18% of people sit in a vehicle for six hours or more a week.
BOA council member Danny Williams said: "While most of us are aware that seatbelts save lives, it's fair to say that the majority of us don't know that the way we sit in a vehicle also plays a huge part in our safety and wellbeing.
"The position of the head restraint, how far or close we sit to the steering wheel and how long we spend sitting at the wheel without having a break can cause long-lasting neck and back injuries."