The speedometer in my '85 300D is way off. My actual speed (checked with GPS) is 50% higher than the speed indicated (reads 40 actually 60, reads 50 actually 75, etc.) My odometer and tripometer work perfectly as checked with both GPS and mile marker signs; literally less than a tenth of a mile off for over 30 miles... The speedo needle moves normally and returns to zero when stopped. This one is beyond me, I thought the speedomoter was mechanical. Does the amplifier on the back effect indicated speed?