Wheel Collider Stops Accelerating

I'm having a problem with wheel colliders reaching a maximum velocity much sooner than it seems like they should. If I apply a constant motorTorque to my wheel colliders they accelerate to a certain velocity and then stop accelerating when I would expect them to accelerate indefinitely. It seems there must be some kind of velocity-based force opposing the wheels but I can't seem to find what it is. I've tried turning down the damping on the wheel colliders and the drag of the rigidbody they are attached to is 0. I've tried making just a minimal car with a box and two wheels and even when it's falling in air, the wheels stop accelerating at a relatively low rpm.