The M113 with the 75M engine was fitted with a TX-200-2 series transmission, featuring 6 forward speeds and 1 reverse speed. The M113A1 and M113A2 with the 6V53 engine was fitted with the TX-100-1 transmission, featuring 3 forward speeds and 1 reverse speed. The TX-100-1 is structurally very similar to the TX-200-2, differing mainly in its simplified gearbox with fewer gears and in the changes made to the valve body to harmonize its operation with the 6V53 engine. The gears available in the two transmission gearboxes are tabulated below. Both transmissions feature a torque converter lockup for improved fuel economy during steady-state driving, i.e. cruising.
| TX-200-2 Gears | Gearing Ratios | TX-100-1 Gears | Gearing Ratios |
| 1 | 5.296 | - | - |
| 2 | 3.8 | 1 | 3.81 |
| 3 | 2.69 | - | - |
| 4 | 1.936 | 2 | 1.93 |
| 5 | 1.39 | - | - |
| 6 | 1 | 3 | 1 |
| R | 6.042 | R | 4.35 |
When the lockup clutch is engaged, the torque converter does not function as a fluid coupling, and the power transmitted to the gearbox is equal to the power available from the engine. The two conditions needed for the lockup to engage are a sufficient vehicle speed, and a low speed ratio (the ratio between the torque converter input and output speeds). The lockup can engage in any gear if the vehicle speed (sensed via the transmission speed) is high enough, but if there is a significant difference between the engine and vehicle speeds, the lockup cannot engage. Without the lockup, the torque converter introduces additional losses, which can be very high at low speeds, greatly reducing the net power available at the gearbox.
For the TX-200-2, the peak torque converter efficiency is 0.85, achieved at a speed ratio of 0.725. At a speed ratio of 0.85, the torque converter ceases to provide torque multiplication, acting only as a fluid coupling with a freewheeling stator, and the lockup clutch engages at a speed ratio of 0.9-0.92. Once the lockup clutch engages, it brings the speed ratio to unity after some initial slippage. Owing to the identical design of the torque converter in the TX-100-1 series, the same figures can be expected.
Note that the basic facts of the torque converter operation and lockup are applicable to other military vehicles with an automatic transmission as well, since lockup clutch engagement is carried out by the same type of mechanism and is dictated by the same criteria in other automatic transmissions with a lockup. For a technical description of the TX-200-2 and TX-100-1 transmissions, refer to "Direct and General Support Maintenance Manual for Transmission Assembly, Automatic, Model TX 200-2A, ..." and "Direct And General Support Maintenance Manual Transmission, Automatic, 2520-066-4240 (Allison Div., Gmc Model TX 100-1)".
In the M113 series, the torque converter is working at a high speed ratio for almost the entire time that the vehicle is accelerating hard from gear to gear. This is particularly true when accelerating from a standstill, where the speed ratio will inherently be at its largest since the vehicle is stopped. The engine will be able to reach a fairly high crankshaft speed (entering its powerband) and begin generating most of its rated power relatively quickly, but the torque converter must cover the gearing range needed to equalize the high crankshaft speed to the low gearbox speed. In doing so, the torque converter spends most of its time in the low end of its efficiency range between 0% efficiency and 85% efficiency. This is illustrated in the graph below, representing a generic single-stator torque converter for automobiles. On average, when accelerating from a standstill, most of the engine power is wasted as heat in the torque converter and gearbox before it reaches the drive sprockets.
The M113, M113A1 and M113A2 all behave very similarly in this regard despite the large difference in the number of gears available between the TX-200-2 and the TX-100-1, as the TX-100-1 transmission could afford the simplification to only 3 gears thanks to the much higher torque of the 6V53 engine. In the original M113 with the TX-200-2 transmission, having 6 forward gears did not mean that the lockup would be engaged for longer periods even though the gearbox covers a wider gearing range, since the 75M engine put out ~30% less torque than the 6V53. This difference was enough that the M113A1 apparently achieved noticeably quicker acceleration than the M113, with a 0-32 km/h time of 10.5 seconds on concrete when combat-loaded, compared to 12 seconds achieved by the M113 when combat loaded, independently verified by Soviet testing. This is the inverse of what normally entails from having more gears in an otherwise identical gearbox, as more gears would allow an engine to remain high in its powerband and accelerate more rapidly.
When moving cross-country, overcoming obstacles or climbing slopes, high losses to the torque converter become unavoidable due to the high torque required to move the vehicle at any given speed, which the gearbox alone is unable to provide. For instance, in the table below, it can be seen that very high driveline losses are incurred at progressively steeper slopes relative to the baseline figure on flat ground. Note that driveline efficiency refers to the efficiency of the entire drivetrain, from torque converter, gearbox, to final drives. Given that the gearbox and final drives are mechanical and their efficiency varies only slightly with speed, and the drop in engine speed at steeper slopes is small compared to the drop in road speeds, the drastic loss in driveline efficiency at steeper slopes can be attributed to the torque converter and its low efficiency at low speed ratios. A comparable mechanical transmission with a clutch can be expected to transmit around 90% of the input engine power under the same circumstances, assuming that spur gears are used.
When other sources of power loss are included, such as cooling losses, intake pressure losses and exhaust backpressure losses, the actual power available at the drive sprockets in vehicles with a torque converter can be as low as a third or a quarter of the gross engine power when driving on difficult terrain. This results in a slower vehicle speed, particularly when climbing hills, as the table above shows. The huge amount of heat accumulated in the torque converter in such circumstances is also a detriment, as transmission cooling can become an issue. An example of this drastic difference in net power under identical conditions can be found in U.S comparative testing of a Centurion against domestic tanks, where the Centurion demonstrated quicker acceleration and vastly quicker hill climbing speeds on the straight-line highway slope and hill tests compared to the M26, which cannot be credited to its marginal advantage in gross power-to-weight ratio (11.6 hp/ton vs 10.6 hp/ton). That said, torque converters provided the advantage of engine stall protection and ease of starting from a stop on steep slopes, allowing even inexperienced drivers to work effectively, whereas inexperienced drivers handling a manual mechanical transmission might outright fail to start their vehicles after stopping on a slope, especially onunderpowered vehicles.
The main technical advantages of a torque converter in military tracked vehicles are to moderate the engine speed under varying loads from terrain irregularities and provide dependable hill starting. Indeed, from a technical standpoint, torque converters provide many advantages that are applicable to civilian and commercial vehicles, including tractors, agricultural prime movers and construction vehicles, such as allowing the driver to smoothly change the speed of the vehicle under a very high load (when bulldozing, ploughing, etc) without requiring a gear change. However, for most military vehicles, such advantages are not relevant to their intended role. This is one explanation for the ostensibly peculiar situation in the USSR where many tractors and dual-purpose heavy trucks were equipped with torque converters, while all tanks and most other tracked and wheeled vehicles were not.
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