A friend found a 74 R-Sport cluster for my 81 242GLTurbo, and in the process of cleaning it up and preparing to swap it into the car I noticed that the odometer adds one mile every time the trip odometer is reset (its the turning knob, not the simple push kind). It adds the mile about when the trip odo gets to 7s or 8s. I'm not sure, as I know next to nothing about speedos, but can anyone give me any feedback. Will this go away when I connect the speedo cable? Does it call for a rebuild? Ideas on how this works?

Speedo is a 130mph made in Feb 80, and shows 81, 510 miles, but it only shows tens of thousands of miles, so actual mileage may be much higher. And speaking of that, is there a version availabe that shows hundreds of thousands of miles, or could a shop add a place for the extra digit? Doubtful I'm sure, but I know not much more than the cable spins and makes the needle thingy move. But I do know it would look nice in my car...

Thanks all

Logan