If you remove the gauge from the back of the Speedo (there's 2 screws holding it in), have a look at the face;-
just below the FULL, 1/2 and EMPTY marks you'll see 2 dots , they are fine and not easy to see, don't sweat it though if they have gone. On these particular gauges they line up close enough to the actual Face marks. On some of the 52 mm gauges, they don't.
If you flip the Gauge over, you'll see two Plugs on the back - these ones were cork, but some are rubber (and some are dust!)
Carefully pry the covers out and you'll see to slot 'headed' adjusters under them
As per the Photo above, one adjusts for the FULL Indication and the other for the EMPTY Indication.
Be aware, each does affect the other end. Ideally, if you have some time, it's best to do a full adjustment here.
To do this, you'll want a 33 Ohm 5 Watt Resistor, a 12 Volt DC Power Supply and take the Voltage Stabaliser from your Speedo.
Connect the 12 Volts to the Stabaliser with the -ve to the Case of it and the +ve to the B terminal.
With the Gauge NOT connected, adjust the 'EMPTY' adjuster until the Needle is on EMPTY. CAUTION:- The Adjusters only need a very small arc of movement.
Connect the -ve from the Power Supply to one terminal of the Gauge and the other Terminal of the Gauge to the 33 Ohm Resistor, the the other side of the Resistor to the I Terminal of the Stabaliser.
It will take around 1 - 2 minutes, for the Gauge to stabalise and it should now lay between the 2 dots at the Full end of the scale. Adjust the FULL adjuster until it does. Give it a good minute between adjustments to let the Gauge Stabalise.
Once that's done, disconnect the Gauge, give it a good 2 minutes and check again at the EMPTY mark.
You'll need to go back and forth between these 2 adjustments a few times because, as mentioned, adjusting at one end does have some impact on the other end too.
Assuming the Sender and the Voltage Stabaliser are good, it should now read correctly.