I just swapped out the 110 speedo for a 145 speedo in my '90 Iroc but the new speedo isn't working. It's not totally dead, it'll read 10-15 mph when I'm actually going around 40 or so. Everything else seems to work fine, tach. seems pretty good, other gauges work, lights light up.

What could cause only the speedo not to work properly? Do I need any other parts to make this swap function properly? How should I go about trying to make this work?

Thanks.

Brian