According to the Zaloga book on the BT, the trackless mode was primarily to save fuel and reduce track wear, not to increase speed. Obviously, however, road movement was faster. To be honest, the Soviet Union had plenty of steel and fuel, and a paucity of rubber, so track wear as opposed to road wheel wear probably didn't really make any difference to the tankers. Switching between the drive system was extremely difficult and required training as the steering was different. Christie loved the idea, but I don't think it ever really worked in an operational sense, as did not the "flying" air drop version of the BT. However, through all the upgrades (BT-2, BT-5 and BT-7) they did keep the dual drive, so at some point they must have used it or they would have done away with it as it added a lot to the complexity and cost of the BT (and reduced its reliability.) But my guess is that in 1039 to 1941, when these were used up, they never had the tracks taken off.