Men thinking body size will allow them to beat the morning-after breathalyser is the height of folly, according to a survey.
Tests show that height and weight of drinkers has little or no bearing on their fitness to drive the day after an alcohol-fuelled evening.
Yet 77% of drivers reckon body size is important when it come to how quickly the body processes alcohol, a poll from Direct Line car insurance showed.
And 70% believe gender is also a vital factor in determining whether someone is legally fit to drive, said the poll which was also conducted by breathalyser company AlcoSense.
The two organisations joined the Transport Research Laboratory for an experiment.
A man weighing 11st 6lb (73kg) and just under 5ft 10in (177cm) tall and a woman weighing 9st 6lb (60kg) and 5ft 5in (165cm) were tested after consuming the same amount of alcohol.
Tested the next morning, the woman was well below the permitted alcohol level while the man was over the limit.
The survey also found that 25% of drinkers knock back lots of water before they go to bed, while 8% eat large quantities of food after a night's drinking.
Next day, 6% have a big breakfast and drink lots of coffee before driving.
Simon Henrick of Direct Line said: "Drivers are taking a huge risk if they are relying on rough calculations and unproven theories to see whether they are safe to drive their car the morning after the night before. Our study shows that, despite what people may think, there is no magic equation to work out if you are safe to drive the following morning."