My speedo in my s5 is off by ~40km/h at 100km/h. (so it reads 60 when im going 100) and its usually off by around 40% all the time.

Wondering if this is a speedometer issue or a transmission issue?

 

check the plastic gear at the trans.it could be worn out.
Take the cable off at the trans,then undo the 10mm bolt,,the whole assembly will pop out(watch for oil).

 

hopefully not, i hear they are expensive :P

 

I had this on my old VW Fox. Never any transmission issues.

 

Similar problem and solution (post 15): https://www.rx7club.com/showthread.p...ht=speedometer

 

Wow, thanks that answers alot of questions

 

In my case there was excessive grease/oil built up between the copper plate and the magnet plate causing too much drag between them and the speedometer read too high. I suspect in your case that there is dirt and grease built up on the speedometer shaft causing drag there so it doesn't rotate far enough and reads too low.

Remove and disassemble the speedometer, clean the shaft that holds the needle where it goes through the bushing and verify that the spring isn't all messed up for some reason (there is a clock spring that returns it to the 0 position). It should spin with no resistance except the spring when it is fully cleaned.