Slipping speedo output "shaft"...?

[Big Yellow]

I have a 230SL with 4-speed manual transmission (1964). Speedometer is stable (not jumping) but not accurate. It goes up to 50-60 km/h but doesn’t go any further however fast I’m driving. I was thinking it’s the speedometer that needed servicing, but thought I’ll check the trip and odo meter against a GPS counter first. The distance shown on the trip/odo was only 75% of the real distance. I have changed the speedo cable, which makes me think it’s the output from the gearbox that is slipping somehow? Anyone has experience on this?
Best regards,
/ Vidar

[Benz Dr.]

I've seen this a number of times. Easy way to test is to remove speedo cable at the trans and insert a small flat bladed screwdriver into the drive. Turn the drive shaft which should make the screw driver turn. If it doesn't turn, or has little pressure to turn, the output flange nut is loose. You will need a special pin socket to tighten it.