You don't really say what you mean by "ability" here. But let's presume a definition of driving ability includes "operating a vehicle in a safe manner." So if we may judge by who has lower auto insurance rates, and if you think safety is an important component of skillful driving, then women may actually be the better drivers:
According to information collected by InsWeb, the median car insurance rate for women is about 9% lower than the rate for men.
The national median rate for women is $698 for a six-month policy. For men, it's $765.
In fact, according to the linked table, in only one state in the U.S. (Iowa) did women pay (slightly) higher premiums than men.
There are other factors considered in the study, including size of car, relative horsepower, etc. But insurance companies make money by matching premiums to the size of the risk. And clearly they think women are a better bet to be good drivers.