  We compare the Baryonic Tully-Fisher relation (BTFR) of simulations and
observations of galaxies ranging from dwarfs to spirals, using various measures
of rotational velocity Vrot. We explore the BTFR when measuring Vrot at the
flat part of the rotation curve, Vflat, at the extent of HI gas, Vlast, and
using 20% (W20) and 50% (W50) of the width of HI line profiles. We also compare
with the maximum circular velocity of the parent halo, Vmax, within dark matter
only simulations. The different BTFRs increasingly diverge as galaxy mass
decreases. Using Vlast one obtains a power law over four orders of magnitude in
baryonic mass, with slope similar to the observed BTFR. Measuring Vflat gives
similar results as Vlast when galaxies with rising rotation curves are
excluded. However, higher rotation velocities would be found for low mass
galaxies if the cold gas extended far enough for Vrot to reach a maximum. W20
gives a similar slope as Vlast but with slightly lower values of Vrot for low
mass galaxies, although this may depend on the extent of the gas in your galaxy
sample. W50 bends away from these other relations toward low velocities at low
masses. By contrast, Vmax bends toward high velocities for low mass galaxies,
as cold gas does not extend out to the radius at which halos reach Vmax. Our
study highlights the need for careful comparisons between observations and
models: one needs to be consistent about the particular method of measuring
Vrot, and precise about the radius at which velocities are measured.
