[KenWittlief]

Tristan has tossed another curve into the discussion.

A non-slipping tire on carpet is static friction, if you are trying to calculate the pushing force of the robot - how much force can it apply to another object, or how fast can it accelerate without spinning its wheels, then you are looking at the static friction of the tire on the surface

but for a motor PID control loop, you dont care about that. You want to know how much friction the drive train is seeing as the motor spins (as the robot moves across the floor). That is the rolling friction of the wheel, and the kinetic friction of all the bearings and gears in the drive train.