The Smiths electric tach from a 1965-1967 MGB is originally wired for positive earth electrical system. If the electrical system is switched to negative earth this tachometer must be converted to negative earth. The following instructions are offered by John Twist of University Motors Ltd:

Two changes are necessary to convert your electric tach to negative earth:
A) The wires must be reversed at the "white wire loop" at the back of the unit
B) The power and earth connections must be reversed inside the case

A) The wire in the "white wire loop" comes from the key switch and travels to the hot side of the of the coil. Referring to the illustration below, select one of the wires and tag it with two pieces of tape for identification. Then, cut the wire between the pieces of tape, and cut the other wire to the same length. Reverse the connection (now there is one piece of tape on each wire) and solder them (remember, this is the power lead for the coil and is unfused). Tape the connections carefully. When later replacing the plastic block on the back of the tach, ensure that the metal band around the block is carefully positioned. This is a necessary part of the electromagnetic pickup.

B) To reverse the power wire and earth wire inside the unit, it is necessary to remove the chrome ring, the glass face and the glare shroud. The chrome ring is usually removed with great difficulty by rotating until the tabs can fit through the slots in the case. Remove the two screws on the back of the unit that hold the internals to the case (not the two whose heads fit in holes in the case), and allow those internals to drop carefully into your hand. Don't bend the needle! The spade terminal is the power connection. Just next to this is the earth connection. A resistor is soldered to one of these connections, and a green wire to the other. Unsolder these ends of the green wire and the resistor from their current positions. Re-solder the green wire to where the resistor was connected, and the resistor to where the green wire was connected. Reassemble the unit after cleaning the glass.