Before I explain the problem, I'm going to give you all the information.

88 L98, 700R4. 3.23 rear end gears. I had the speedometer gears changed as the ones that were in it were wrong. It now has a 15 tooth drive, 38 tooth driven gears. Standard IROC tires, 245/50/16 - according to cooper the tire is 25.7 in diameter.

Now to get to the problem. The speedometer reads 10MPH too slow up to from what I've been able to tell even at 60MPH. However, it seemed to be 15MPH off at 75MPH.

Before I got the gears changed - it read 10MPH too slow too, having 17 tooth drive, 44 driven gears. The odometer was spinning like crazy though.

I originally broke the needle, and replaced the needle and shaft with a donor speedometer. Those were the only "new" parts I used. They were out of a 110 speedo.

There is a black hole of information on the design and workings of these speedos. Did the same design get used on other vehicles? I wonder if there is something wrong with the magnet, if the clockspring is somehow bad or the wrong rate? My stepmom told me they had lots of issues with the speedometer, and at one point it quit working altogether. I'm coming to the end of my rope on this one.