More than six months after new federal rules went into effect that prohibit banks from charging consumers overdraft fees unless they “opt in” to such an arrangement, government data are lacking on how this has changed banks’ bottom lines.

“This is the most expensive form of credit, and consumers most likely to use it are the most vulnerable. You’d think regulators would want to know exactly how much revenue banks are taking in,” says Jean Ann Fox, director of financial services with the Consumer Federation of America.

The federal regulations came after years of controversy over banks’ practice of ...