Here's a mechanism for controlling your LEGO tanks that may be considered a good alternative to subtractors. It splits a single driveshaft in two and then puts each of these two shafts through a small 3-speed remotely controlled transmission. This effectively allows to drive at two different speeds and to turn with two different radiuses. Itβs also fairly easy to add a transmission and/or a piston engine between the drive motor(s) and the mechanism, and β unlike with a subtractor β they will work both when driving straight and when turning.
The primary advantages of this mechanism are realistic controls (with a PF remote you can operate levers just like in a real T-55 tank) and ability to use a transmission and/or a piston engine at all times. There are no differentials needed, which is another advantage because their inner gears can be broken under high torque. The disadvantages include the need for two PF Servo motors to control the mechanism and a larger size and higher number of specialized pieces when compared to a subtractor.
Free building instructions: http://sariel.pl/downloads/