Roamerdrive DS-B

Our overdrives are manufactured in British Columbia, Canada and Perth, Australia. 

Garage Therapy Overdrives fit most Series and Defender models as well as some Range Rover and Discovery models with an integrated, LT230 or LT230Q transfer case. This includes:

• Series I, II, IIA, and III
• Stage One V8 (including ex-military models)
• Defender 90, 110, 127, and 130
• Perentie (ex-Australian Army)
• Range Rover Classic
• Discovery 1 and 2
• 101 Forward Control

If you’re unsure about compatibility, contact us for more details. 

Garage Therapy Overdrives reduce engine RPM by 28%, making highway driving quieter and more fuel-efficient. They also reduce engine and drivetrain wear, act as a splitter gear for better performance on hills, and improve low-range capabilities in off-road conditions like sand and desert terrain.

The LT95 overdrive’s power-handling capability is governed primarily by torque rather than outright power. Its planetary gear assembly is designed to manage torque levels on par with those of the LT95 main gearbox. Planetary systems distribute loads over multiple gear teeth, resulting in notable strength and durability for their size. Nevertheless, the limiting factor in overall drivetrain integrity remains the main output shaft of the gearbox.

Under high-torque conditions, lateral forces produced by the transfer gears can cause excessive main shaft deflection. This deflection leads to uneven loading across the transfer gears, potentially resulting in gear tooth surface damage and, in the case of LT95A gearboxes, may cause failure of the intermediate transfer shaft retaining bolt.

To counteract these issues, this LT95 overdrive design includes an additional main bearing on the output shaft and an extra needle roller bearing in the coupling assembly, both of which mitigate shaft flex and enhance the rigidity of the transfer gear assembly.

Despite these reinforcements, shock loading—rather than peak engine torque—remains the primary cause of drivetrain failure. Consequently, proper driving technique is crucial in preserving drivetrain longevity under off-road conditions.

Large four-cylinder diesel engines such as the Isuzu 4BD1 frequently impose higher stresses on drivetrain components than high-torque gasoline V8s, due to their non-overlapping power strokes that generate higher peak torque. Engine manufacturers partially address this by increasing flywheel and clutch assembly inertia. However, adding an aftermarket turbocharger without a corresponding increase in flywheel inertia can intensify stress on drivetrain components.

Aside from ultimate strength concerns, elevated operating temperatures are commonplace due to the overdrive’s position behind the engine and main gearbox, where airflow is restricted. The LT95 transfer case itself is known to operate at high temperatures, often exceeding 100°C. With increased power demands, prolonged high-speed operation, and high ambient temperatures, the additional heat generated by the overdrive can push operating temperatures beyond the design limits of standard gear oil, leading to lubrication failure.

Even installing a finned transfer case cover may be insufficient to maintain safe oil temperatures in these conditions. In such scenarios, an external oil cooling circuit is recommended. Failures resulting from lubrication breakdown are not covered under our warranty.

Unlike a high-ratio transfer case, a Garage Therapy Overdrive can be engaged or disengaged at any time, reducing RPM by 28% without affecting low-range performance. High-ratio transfer cases make permanent changes to your gear ratios and often reduce off-road capability. They typically reduce RPM by only 15% or less and can make it harder to climb hills or handle heavy loads. Our overdrives are also easier to install and can be moved between vehicles if needed.

Most owners can install the overdrive in about 3 hours using standard tools.

• On Series models, a small hole must be drilled in the transmission tunnel for the shift lever.
• On Defenders, installation is a bolt-in job with no modifications to the bodywork.

Garage Therapy Overdrives are designed to be low-maintenance. They use transfer case oil, so there’s no separate oil reservoir to maintain. There are no dynamic seals, minimizing leaks, and they can be disassembled with common workshop tools. Parts like bearings and synchros are standard Land Rover components, making repairs simple and affordable.

Only use API GL4 75W90 oil in the transfer. Do not use GL5 or GL4/5 oils as these are designed for hypoid gear sets and often contain additives which may be corrosive to the internal bronze components used.

Only use API GL4 75W90 oil in the transfer. We recommend fully synthetic oil for high temperature climates or where high loads and extended high speeds are anticipated.

Do not use GL5 or GL4/5 oils as these are designed for hypoid gear sets and often contain additives which may be corrosive to the internal bronze components used.

For a comprehensive list of oils please see this document.

Yes. On Series vehicles, the SS model can handle popular upgrades like the 200Tdi engine. In these cases, the SX-COV transfer case oil cooler is required. On Defenders, the DS model can handle up to 380 Nm of torque. For higher power outputs, it’s essential to install the DS-COV oil cooler and monitor oil temperatures to prevent overheating.

Garage Therapy Overdrives are highly reliable for demanding expeditions. We recommend packing a basic seal and gasket set and your original drive gear and cover in your spares kit. Failures are extremely rare, but seal kits are available from our distributors if needed.

The shift mechanism is fully mechanical and easy to use.

• To engage: Press the clutch and move the lever forward.
• To disengage: Press the clutch and move the lever back.

The lever is mounted on the transmission tunnel for easy access.