Posted a bunch of links here that might help
lotus-electrical-f38/speedometer-rebuild-t29691.htmlI had the same issue. Took the tach apart and exposed the adjustment screw. Got it to work for a while, but eventually went for full repair/conversion by a specialist. The components in the tach appeared to be well past their best before date and the tach was unstable with temperature change. Overall advice is either fix it or send it off, not worth trying to adjust, but it is possible.
If you want to take on the tach conversion yourself, this thread looked very promising.
elan-mods-f31/rvi-tachometer-conversion-t26919.htmlTrip Odometer is set with a remote knob. Cable attach's to the back of speedo similar to the speedo cable. Knob locates on dash bracket by the door light switch. Is this missing from your car?
Check odometer is reading correctly first using a test section on the highway, mile posts, etc. You can see turns per mile (TPM) of speedo head in small print on speedo face. TPM can be changed by a specialist if required. I needed to do this after changing transmission and diff ratio.
If your odometer is basically correct now, the speedo head needs repair to read speed right; I just sent it off as it is very delicate and needs a test bench to fix correctly. There are pages on the net that discuss home repair and calibration with a pistol drill, but i didn't bother.
William raises an excellent point regarding transmission gear. Just check the odometer reading to see if you are close first. If the odometer is right the transmission gear is right. I think speedo will typically read high speed as the internals weaken, but it has been a while since I read up on the topic.
Use David's excellent spreadsheet to calculate things for your exact transmission, diff ratio, tire size, etc. I adjusted the tach to GPS speed readings at calculated revs for my setup. This worked very well, but instrument would not stay adjusted.
Can provide more detail if required, but ended up sending both instruments to a specialist anyway.
HTH