Originally Posted by tabetha

The wrong sensor(resistance) will certainly give a different reading, they can usually be told apart from the coloured ring on the base of the stud that the connector goes on, not sure what colour is correct, but cossie/sierra/ford guages of this era not the most accurate in the world, maybe try a new sender from matt lewis, or just put up with it.
The early mk1 gauges actually had an(mechanical) adjustment on the fuel and temp gauge readings.
tabetha