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How the mechanical governor speedometer works

The drive from the road wheels via the gearbox or back axle is transmitted to the speedo head via a flexible drive.

This drive is passed to the mechanical governor. The governor is simple mechanical assembly. As the rotational speed of the governor increases, it causes a linkage to move. This linkage is connected via a gear to the needle. As the road speed increases so the needle moves round the dial giving an indication of the road speed.


Using the block diagram to show what is happening as the central shaft (1) driven by the flexible drive revolves, the weights (2) try by centrifugal force to fly outwards. This outward motion is transmitted via the linkage (3) to the sliding collar (4). As the speed increases the sliding collar is pulled (in my drawing) to the left. The needle linkage is connected to the sliding collar, and as the collar moves drive is transmitted via a quadrant gear and a geared pinion to the needle.


Let no one call me a gifted illustrator, but the diagrams (left) should show how the speedo works. Please note that this diagram is for the Humpback governor (see below). The Flatback drawings will appear soon. I have taken quite a bit of artistic licence with this drawing.

Humpback gov schematic


There is in the mechanical governor speedometer, a direct mechanical linkage between the road wheels and the speedometer needle. In the NEMAG there is no direct mechanical linkage. Thus it might be assumed that the accuracy of the mechanical governor speedometer is greater than that of the NEMAG. This could well be true but care must be taken, even correctly calibrated speedometers only give an indication of road speedo, speedometer vary plus or minus between 5% and 10%.

To add a few more details on to the block diagram. The action of the sliding collar (4) is restricted by a compression spring (GGG). Various type of mechanical governor speedometer have a springs of differing compressive strength. Smiths provided a chart which linked the Speedometer to the correct spring strength. A spring strength gauge, tool number SR/D 183, was produced to assist the Smiths technicians during service. Sadly most of us do not have such a tool.

Using the examples as above the Letter N speedometer uses a governor spring designated P.4087. The Speedo letter D having a different TPM we might expect the spring to be different, but it also uses the P.4087 spring. The P.4087 spring is marked with a Letter c . There are only six different spring strengths.

If there are only six different spring strength as there are at least 37 differing MA types then there must be other potential variable in the speedo drive system and there are.

hump and flat back


There are two major types of Governor speedometers, as far as I am aware there is no correct name so we call them the “FLATBACK” and the “HUMPBACK”

The photograph (below) shows the internal governor assembly the top one is the humpback and the lower one is the flat back.

flat and hump internals



The photograph of the cases (right) shows the flatback top and the humpback lower.




The operation of the flatback governor speedo is a bit different from that of the humpback

In the flat back the needle or pointer is connected via a train of gears and linkage to the brass rose or ball joint seen at the top of the speedometer. When the speedometer is at rest, the ball joint is in an extended position.

flat at rest


When the shaft is turning at speed the governor weights, under centrifugal action, are ‘spun out’, as the weights move out, an internal linkage draws the brass joint back into the speedo body. The movement of the brass ball joint is connected by a geared linkage to the pointer as the revolutions of the speedometer shaft varies, so does the centrifugal action and so in turn, this variable movement is shown by the pointer on the dial.

flat at speed

The geared shaft at an angle to the main governor shaft, is the drive to the odometer.

There other differences in the speedometers;

The trip operation mechanism ( If fitted). If a trip is fitted they can be operated either through the front of the dial, or operated via a lever at the back of the speedometer.

The other major difference is the way that the flexible speedometer cable is connected to the speedo. The first photograph at the top of the page shows a clevis and pin fitting, the lower photographs of the governor types show male square shafts, which fit into female sockets on the speedo cable.