I got readings close to the same when going through my brand new gauges, went through all the trouble shooting as described be strenk and finally found out that the little dimples (grounds for the gauges) on the speedometer housing were not quite grounding. I discovered this because I scrapped them with a screwdriver and cleaned them up with a wire brush and viola! it all worked. Try that! and hopefully it works.
1984 CJ7 4.2 L 258 Motorcraft 2100 Carb Nutter Bypass Done