Here maybe even more of a hint:
http://ifirobotics.com/images/wheels/IMG_0152.jpg
is that the...kit chassis?
Oh and on the andymark wheels are 4", 5", and 6" outside diameter whereas the IFI wheels are 4", 6", 8". Other changes that I noticed where the differences in the Tread with (a key player in friction)
Outside Diameter-Width (of tread)
AndyMark
4"-1.3"
5"-1.8"
6"-1.8"
IFI
4"-1", 1.5", 2"
6"-1", 1.5", 2"
8"-1", 1.5", 2"