  Biomolecular motors use free energy to drive a variety of cellular tasks,
including the transport of cargo, such as vesicles and organelles. We find that
the widely-used `constant-force' approximation for the effect of cargo on motor
dynamics leads to a much larger variance of motor step number compared to
explicitly modeling diffusive cargo, suggesting the constant-force
approximation may be misapplied in some cases. We also find that, with cargo,
motor progress is significantly more precise than suggested by a recent result.
For cargo with a low relative diffusivity, the dynamics of continuous cargo
motion---rather than discrete motor steps---dominate, leading to a new, more
permissive bound on the precision of motor progress which is independent of the
number of stages per motor cycle.
