My initial idea was to use a rheostat and what you say does sound similar to that. Might as well give it a shot will I can get specs for the indicators.
Meanwhile, I found this. http://www.allaboutcircuits.com/vol_3/chpt_8/7.html
Any Comments about this tranconductance circuit ?
Really appreciate the help!
It does not matter what resistance value Rload is, or how much wire resistance is present in that large loop, so long as the op-amp has a high enough power supply voltage to output the voltage necessary to get 20 mA flowing through Rload.
Same requirement. Rload of that circuit is equivalent to the internal resistance value of the speed indicator. If Rload is unknown then the minimum value for the op-amp power supply is also unknown. Have you considered taking a ohm meter and just read the speed indicator's resistance?