I could see that there's nothing else causing any friction other than the weight of the motor and/or probably the bearing of the axle. I might be completely wrong in my assumptions...
Consider a simple motor connected to a load. The load is the causative effect for friction but it is rarely described in this fashion.
An insufficient current through the motor will not cause any motion because the load is providing a static friction. In other words, the motor is producing torque but the load is not moving. You need to apply a minimum force before the load can move.
Once the load starts moving, the static friction disappears and dynamic friction comes into play. Usually, you need a greater toque to start but a slightly smaller torque will be sufficient to keep the load moving (dynamic friction is a different beast altogether).
For a vehicle, dynamic friction is caused by rolling friction at the bearings, air friction, friction at the contact surface etc.
Usually for a loaded motor, the inertia of the rotor is only a small part of the friction; a well made motor can start will a small current (the friction at the bearing is relatively small). But when you have to start the motor with load on, you need to apply far larger current to start.
Internal combustion engines (say a diesel engine) cannot start on load; we need to have a clutch for that. Electric motors do not need a clutch. But an electric motor need excess current to start on full load (say 20-50% extra) and the motor should be able to withstand the higher current for a shorter period.
AC motors (say motors you find in house fans) have poor starting torques; here the brushless motors excel. To start an AC motor on load, the motor must be overrated but then it will take less power when running normally. DC motors are best when you have lots of start and stops.
To compute the current, first determine the load and compare that with the rated power of the motor. Use 50-60% extra current to start but reduce that to 80-90% of the rated capacity once the motor picks up speed.