There is a major difference between unit speeds in game and IRL. The game speeds are based on the tactical speed of some representative vehicle, so light and medium armor has different speeds. In reality, the (strategic) marching speed of all motorized units (all armor, mot, mech, etc) was about the same.
That said, however, in game the game the number of meaningful choices is very limited. It is nice to try to adopt various semi-historic compositions, but the only parameters that really matter are
softness and
speed.
You want to keep your
softness as low as possible in the
34%-67% range for the
combined arms bonus, and your speed as high as possible. This means that you want to
avoid including slow combat or support units in your division.
In the late war period, Arm and Mot have matching speeds (9 and 8.7 respectively) and L. Arm and Mec do as well (11 and 10.4 respectively).
See
http://www.paradoxian.org/hoi3wiki/Land_units for speed listings.
The only support units that can keep up with the Arm / Mot division are AC, SP Art, and SP R Art. However, all of them have a high softness. Thus, by adding them to the best combination of
3 Arm + 1 Mot,
with a softness close to the optimal 1/3, this gets diluted.
The fast division consisting of
2 L. Arm + 2 Mec or
1 L. Arm + 3 Mec
on the other hand can only be combined with SP R Art without loss of speed (but with some loss of hardness).
Experimenting with other support units only makes sense for H. Arm. Here you also have Eng and TD, which actually provides hardness.