[Espeefan]

The big rig and Highlift transmissions both use identical internal gear sets for 1st, 2nd, and 3rd gear. It wouldn't make sense for Tamiya to tool up special gears for the Highlift, so the parts carry over. What is different about the transmissions though, is that Highlift has an added transfer case to the rear, and the gears used in the transfer case portion provide an extra reduction that the rig transmissions lack. There could also be a difference in spur gears or pinion gears as well, but internally, excluding the transfer case of the Highlift transmission, they are identical in design.