I have just found out that my speedometer tells me that I'm going for 100km/h when the actual speed is 90km/h. My car is a 300SL -91. I guess that it's possible to set this up in some way... Does anyone know anything about this procedure and perhaps set a degree of difficulty?? (I checked at 50 km/h too where the difference was approximatly 5 km/h so we are talking 10%)

Spedometer off by 10%
 

My guess is that your tires are not the correct size or the stem of the speedometer needle has been twisted. Has anyone ever removed your instrument panel?... and possibly twisted the speed indicator needle?

I have no chance of knowing if anyone has messed with the stem. There are no papers followed the car though, indicating any possible repairs in that area. I was thinkning about your guess with the tires. When I bought the car it was fitted with 16" tires. My guess is that since the car is a 300SL it has been fitted with bigger tires and I thought that the speedometer was calibrated with those tires. Could that be the case?? In my mind that would result in the speedometer showing a lower speed. I don't know , , Maybe I way off?

The R129 300SL's came originally with 225/55-16 tires.
Twisting the stem of the speedometer needle changed the displayed speed on the older models with the Speedo cables. I doubt that anyone has messed with the speedometer needle on your car (you can check this by taking out the dash cluster; the cluster-parts have to be sealed with a sticker).
Personally I don't think there is a way to calibrate the speedometer...but I am not a mechanic!

greetingz,

My initial question to myself was "How did you test your spedometer? I would take it to an expressway where there are milage markers and then take readings on your odometer as you pass a series of markers...say a reading at each mile and then do an average at 5 miles. If your odometer agrees with the markers then there should be nothing wrong with your speedometer. To throw your speedometer off something significant must have occured over the history of the car. Like a new rear gear box with a different gear ratio.

If your speedometer is electronic (not mechanical cable) you might check the electrical cable connector where it plughs into your gear box and check for corrosion or poor contact.

At first I just had a suspicion that the speedometer was a little off. That made me perform some tests and just as you said I used the marks on the highway that the police helicopters use to determine the speed of vehicles. In sweden the marks are set every 0,5km. I set the tempometer at 100km/h and did several readings, all pointing to 20,8 sek. According to my calculations that would be 86,5 km/h so it's almost 15% off. (However, how I see it, the first 5% is standard deviation that most cars have). Thank you guys for looking into this problem.