Autonomous driving may actually make it more difficult for automakers to meet efficiency standards, according to a new Bloomberg report. That’s because the extra weight of the sensors and computers used in self-driving cars could negatively impact fuel economy. Added bulk could also reduce the ranges of the electric cars many automakers view as necessary to comply with tougher emissions rules.

Over the past few years, automakers have gotten more serious about reducing weight. Lighter materials like aluminum, high-strength steel, and composites have become more commonplace, and it’s rare to get a press kit for a new car that doesn’t mention weight savings. This is largely to improve fuel economy in anticipation of stricter standards, like the 54.5-mpg Corporate Average Fuel Economy (CAFE) fleet average automakers must achieve in the United States by 2025.

But autonomous driving is undoing a lot of that work. Self-driving cars are covered in cameras and lidar and radar sensors. Their systems also draw a lot of power, more than current 12-volt electrical systems can provide.