93 600 sel speedo error

[mithra]

Recently joined the ranks of the faithful with a 93 600 sel (egads). Curiousity lead me to check the speedo/odo combo with gps and mile marks. The odo is accurate to less than a 1% error over 800 km. The speedo is optomistic by a consistant 7 to 8 %. ( 120 km/hr indicated is actually 112/113 km/hr) The questions are : is the speedo electric? , is there an adjustment?, are the odo and speedo fed by different inputs? The present tires are properly sized 235/60/16. I have reviewed many threads/forums, but none have quite answered the questions.

Mithra

[stevebfl]

One speed signal for both odo and speedo.

The 140 chassis originally had a mechanical odometer but all replacement inst clusters now have digital odometers. The speedo is adjustable internally on a bench. An input is fed the device to make 60mph (don't know the speed in km) using a signal generator and the signal frequency given by the calibration constant on the back of the instrument, K=xxxx. If you have the right odometer as you say, the speedo just needs recalibration by a speedo shop.

[mithra]

Thank you Steve, for your reply. Is this an adjustment that could be made by a reasonably competent individual with electronic equip at home, (scope, signal generator, etc, etc) or is this a shop only issue ?