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Adding Overdrive to an MGB

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I’ve written about overdrives numerous times but not specifically about adding overdrive to an MGB. A post on Facebook made me realize some additional information would be helpful. Hence this additional article specifically on adding overdrive to an MGB where the car doesn’t have overdrive.

Here’s a quick list of considerations with a focus on 1968 to 1980 MGBs. I have very limited knowledge on the earlier cars with the narrow transmission tunnel. The early cars also had the 3 main bearing engines and 3-synchro gearboxes, or 3-synchro gearbox and D-type overdrives.

  • Overdrive gearbox
  • Speedometer implications, potentially
  • Wiring harness and switch
  • Speedometer cable
  • Drive shaft

Overdrive Gearbox #

At the risk of “stating the bleedin’ obvious” you will need an overdrive gearbox. If you somehow manage to obtain just the overdrive unit and the adapter, you’re not going to be slapping it on the back of a standard 4-speed gearbox! Why? Because the main shaft of the gearbox is a very different part between overdrive and standard gearboxes.

Now if you have a perfect condition standard gearbox, and are tempted to try and reuse it, keep the main shaft in mind. You will have to essentially disassemble the standard gearbox and transfer everything from the main shaft over to an overdrive gearbox main shaft. Then reassemble everything and add the adapter and LH overdrive. This is not a trivial task.

Overdrive Gearbox Options #

There are essentially 2 options for the 1968 to 1980 cars. An LH overdrive with so-called black label and blue label variations. The main difference between the two is the gearing for the speedometer drive. Black label overdrives were originally paired with a 1280 turns per mile (TPM) speedometer. The blue label version was paired with 1000 TPM speedometers. I’ve already documented the differences in much more detail. Please refer to my Overdrive Options article. Do not rely on the side fill or top fill bell housing to determine which is which!

Speedometer Implications #

So, clearly, the simple way to avoid the addition of an overdrive messing up your speedometer “accuracy” is to pair the overdrive you obtain or build to the speedometer in your car.

Overdrive Speedometer Gears #

If you are completely refurbishing the overdrive unit, you could conceivably change the gearing inside the overdrive to match the speedometer if needed. As you will see from the workshop manual diagram the main gear (part #86) is deep inside the overdrive. It is not a trivial piece of work to get at this to change it. The gear that this drives and subsequently drives the cable to the speedometer (part #89) would also need to be changed. The parts are different colours between the 1280 and 1000 TPM modes. The later 1975-1980 blue label overdrive units have red coloured gears for both parts. The early 1968-1974 black label overdrive units have white coloured gears. Using Moss Motors as one example of a supplier you can see additional detail. Their diagram part numbers are 20 and 33.

The image is from the workshop manual and is an exploded parts diagram of the LH overdrive. The two gear parts which define the turns per mile have been highlighted.
Workshop manual extract with highlighting of the two elements of the speedometer gear drives

Speedometer Gauges #

I’m going to deliberately stay away from the part numbers of the gauges. For the purposes of this article the focus is only on the turns per mile (TPM). From what I have seen the TPM is shown on the very outer edge of the gauge face. Look at approximately the 5 O’clock position. The part number, for reference, is usually at the 6 O’clock position, also on the very outer edge. This information relates to USA specification cars. The home market cars may be different with different dash styles and gauges.

The image shows a 1000 TPM speedometer gauge with the 1000 marking highlighted and shown larger as an enlarged inserted image.
Example speedometer gauge face from a 1975-1976 MGB showing the 1000 TPM designation

Blue Label Overdrive Into a 1968-1974 MGB #

To align with the black label overdrives that use a 1280 TPM gearing the 1968 to 1974 cars use a 1280 TPM speedometer. Similarly the 1975 to 1980 cars are 1000 TPM speedometers. So if you are trying to put a blue label 1000 TPM overdrive into a 1968 to 1974 MGB, you will want to locate a 1000 TPM speedometer from a 1975 or 1976 car. All these gauges are the same size – approximately 3.5 inches.

Black Label Overdrive Into a 1975-1980 MGB #

However, if you are going the other way around, and putting a black label 1280 TPM overdrive into a 1975-1980 car things may be a bit more complex. If the receiving car is a 1975 to 1976 model, then a 1280 TPM speedometer from a 1968-1974 should be a straight swap. Whereas if you are putting a 1280 TPM black label overdrive into a 1977 to 1980 car, there are no straight swaps. It may be possible to find a larger gauge to fit the 1977-80 (Mark IV) dash that is 1280 TPM. But it will not be an identical swap. The larger gauges for USA cars were never manufactured with a 1280 TPM setup. I recall I have seen a 4 inch speedometer with a 1280 TPM setup. However, I don’t recall what car this was used in or a source for obtaining it.

The image is of a speedometer from a 1977-1980 car dashboard and also has the 1000 TPM marking on the gauge face.
Late 1977-1980 Speedometer with the 1000 TPM marking
A wider angle view of a 1977-1980 MGB dashboard with the speedometer, clock and temperature gauges.
Late 1977-1980 MGB (Mark IV) style dash and speedometer

Speedometer Re-calibration #

If you are particularly keen to keep your gauge, even with a mismatch, then it is possible to have the gauge re-calibrated. I don’t have any recommendations or known service providers to suggest. However, this is a possible option. At one time Nisonger was a firm I knew of for this service but they are not in business any longer as of time of writing.

Wiring Harness and Switch #

There will need to be additional wiring for the overdrive to connect into the main car harness. Also necessary is a switch to turn on and off the overdrive itself. Also note there is a transmission interlock switch (TIS) on the gearbox itself. I do not go into the TIS here as that is covered in some detail in my MGB Overdrive Diagnostics article. Suffice to say you want to test this extensively before installing the overdrive gearbox into the car.

Stalk Switch – 1968 – 1976 MGB #

The same switch for Washers, Wipers and Overdrive is in use on all cars. Whether the car is fitted with overdrive or not, the overdrive switch is part of the stalk multi-function switch. You can easily test this. The stalk will move forwards and backwards. Towards the dash is on for the overdrive. Pulling the switch back towards the driver is the off position.

As a result of this the wiring from the switch to the engine bay is already present. The necessary addition is the wiring from the dash/main harness down to the overdrive solenoid, via the TIS. You can purchase this or fabricate your own as it is a very simple bit of wiring. In the engine bay you you will find in the clump of bullet wire connectors, a yellow wire with a joiner/connector on it that isn’t attached to anything. This is the wire from the switch.

The picture shows where the various wiring harness parts come together and the yellow wire from the overdrive stalk switch has been highlighted.
A 1972 MGB into which a complete new wiring harness was installed. The yellow wire from the overdrive stalk switch is highlighted.
The pictures shows an MGB dash stalk switch with some markup showing the forward and backward direction of motion to turn the overdrive on and off.
An example of the motion of an overdrive stalk switch

Shifter Switch – 1977 – 1980 MGB #

With the late MGB Mark IV new dashboard style, came the overdrive switch in the top of the gear stick/shifter. This does have a reputation for the wires that run up the shifter getting worn and shorting out. However, so far this has not been my experience. If you are adding overdrive to a non overdrive car, the shifter will not have any necessary switch or wiring. The different cap and the necessary switch are available items from parts suppliers.

The actual switch is not a very expensive part. I would suggest keeping a spare on hand.

An image extract from a 1978 sales brochure showing the overdrive switch in the top of the gear stick / shifter
Extract from a 1978 sales brochure showing the overdrive switch in the top of the gear stick / shifter

Wiring Harness #

Before the harness, lets mention an inline fuse. The overdrive circuit is not fused. There are plenty of places that the wiring could chaff and short out. Particularly with the late model on gear stick/shifter setup. So, plan on adding an inline fuse for the overdrive wiring.

You can purchase this sub-harness from a supplier. Alternatively, you may want to make up your own. It is a very simple bit of wiring. Using Moss Motors as a supplier example, they have the harness wiring you may need. Or you can refer to the relevant colour wiring diagram for your car. This will help you make up the necessary wiring.

Speedometer Cable #

This is pretty simple, you need a new speedometer cable. The overdrive speedometer cables are longer because the attachment point on the overdrive unit is further to the rear of the car. In addition to the length make sure your get the correct overdrive cable for the type of speedometer you have.

Drive Shaft / Propeller Shaft #

Depending on which supplier websites you review, there is a suggestion of a different drive (prop) shaft between standard and overdrive gearboxes. I believe this may be more relevant to the early 1962-1967 cars. In my experience with the 1968-1980 cars and the LH overdrive, whatever drive shaft you have should work without changing it. There is a less that one inch difference in length. This is from the output flange of the gearbox/overdrive and the differential.

There is the ability to extend adjust the length of the drive (prop) shaft. So, I would do that first and it should be fine.

Other Materials #

There are quite a few articles on this site for the MGB Overdrive. Try the Gearbox and Overdrive category of the knowledge base to see them all.

I’ve built this website and spent time on the knowledge base to bring together a lot of information I have collected from around the internet. The intent is making it easy to find rather than having to know about and have skills with search engine query terms to get what you were looking for. Please make use of the knowledge base.

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