Comprehensive vehicle insurance premiums fell by 7% in the first three months of 2018, the largest quarterly drop in four years and the third in a row according to research by Confused in association with Willis Towers Watson.
Insurers are not passing on cost savings to drivers, comparison site Confused.com warns.
Reforms to the law on whiplash and a review into how compensation is paid for long-term injuries were thought to be behind the drop.
The number of cars registered to women has risen by 21% since 2007 to 11.8 million, according to the Society of Motor Manufacturers and Traders.
United Kingdom men are now paying £95 more than women, while young male drivers aged 17-20 are still paying the most of any demographic, £2,348 on average, compared to the £1,699 paid by their female counterparts.
Insurers are forbidden by law to set different auto insurance premiums exclusively based on gender.
The average figure for vehicle insurance masks huge differences according to the type of driver.
The Ogden rate is a formula used to calculate compensation for accidents, and a new rate proposed by the government a year ago sparked outrage among insurers and threats of huge premium increases.
The discount - or Ogden - rate is created to compensate insurers for the cost of long-term injury claims. In September previous year, the government backtracked after the insurers said the new formula would overcompensate crash victims. After years of tax hikes and rising premiums, a third of drivers say auto insurance is becoming unaffordable.
"However, it's clear that many motorists are not seeing these savings reflected in their renewal letters, so it's more important than ever to shop around if they want to get a better deal".