Not sure what you have got hooked up. Would need to know exactly what was done for a more exact opinion. But gauges are generally driven by various sensors for different indications. To affect the ECM you would have to afect an input to it, or possible load an output from it to change its proper operation.
I don't know if it is possible to disconnect the spedo to support your opinion as to what might be the problem. It may be a good starting point.