More than a quarter of motorists make up the number of miles they drive each year, putting them at risk of paying too much.
Figures released by price comparison site uSwitch show that 27 per cent of motorists guess their annual mileage when applying for car insurance. This puts them at risk of overpaying or even invalidating their policy.
Rod Jones, insurance expert at uSwitch.com, says: "While many of us are guilty of relying on gut feeling when providing our mileage information when applying for a car insurance policy, misjudging the number of miles you drive in a year can have a real impact on your wallet."
Research by The Mirror found that the difference between stating annual mileage as 20,000 or 6,000 was nearly £70.
No sample size was given for the survey, but worryingly, motorists who understate their annual mileage could find their policy is invalidated – insurance companies can look back at quote histories to see if you've experimented with changing the mileage before taking a policy.
However, there is an easy way to make sure you're not paying the wrong amount. "It pays to dig out your MOT certificates to pinpoint the number of miles you drove last year to help you to provide a more accurate estimate for the one coming up," said Jones.