As many as 8% of drivers incorrectly think the motorway speed limit is 80mph, according to a survey.
This figure rises to 11% for over-65s while 1% believe the motorway limit, which is 70mph, is only 60mph.
While only 4% of women thought the limit was 80mph, as many as 9% of men incorrectly gave the higher figure when polled.
The AA/Populus survey of nearly 23,000 motorists also found that Scots and those living in north-east England were most likely to think 80mph was the right answer.
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, but drivers in the east of England were most likely to correctly identify 60mph as the national speed limit for cars on a single carriageway.
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."