EPA Administrator Scott Pruitt is promising the auto sector regulatory certainty, but his recent decision to revise Obama-era fuel economy standards may create a hazier future as the industry plans investments.

Auto industry suppliers are raising concern that the Environmental Protection Agency’s April 2 determination—a finding that greenhouse gas limits for model year 2022-2025 passenger cars are not appropriate and should be revised—lacks robust data.

And they’re already outlining a worst-case scenario in which negotiations between Trump administration officials and...