As many as 8% of drivers incorrectly think the motorway speed limit is 80mph, according to an AA/Populus survey.
This figure rises to 11% for over-65s. The speed limit is in fact is 70mph.
The survey of nearly 23,000 motorists also reveals that:
- While only 4% of women thought the limit was 80mph, as many as 9% of men incorrectly gave the higher figure when polled.
- Scots and those living in north-east England were most likely to think 80mph was the right answer.
- 1% of motorists believe the motorway limit is 60mph.
- Generally, young drivers were more likely to be correct than those aged over 65 when asked what the the dual-carriage speed limit was (70mph) and also the single-carriageway limit (60mph).
The only speed limit over-65s knew better than 18-to-24-year-olds was correctly identifying 30mph as the limit in a built-up area with street lamps.
However, although fewer 18-to-24-year-olds correctly identified this, they erred on the side of caution with 6% saying they would assume the limit to be 20mph.
Nearly half of drivers in north-west England (47%) and 40% of Londoners did not know what the speed limit on a single carriageway was.
AA Charitable Trust director and AA president Edmund King said:
It is astonishing that 250,000 qualified drivers don't seem to know the motorway speed limit is 70mph.
Some confusion about the motorway speed limit for cars could well be down to a previous Transport Secretary floating the idea of raising the limit to 80mph.
He went on: "There is no reason why drivers shouldn't be able to correctly identify the speed limit of dual carriageways and single carriageways.
"The lack of knowledge around limits just highlights the importance of keeping your driving skills up to date.
This follows a previous AA study that showed that some 17 million drivers would struggle retaking their driving test."