I've been trying to fix this very slow reading speedometer and I can't seem to figure it out.

I know its way off because I can be in 5th gear at 3000rpm and it says I'm going 40 MPH.

It seems like something is causing it not to go as fast as it should.

I've checked the back of.the gauge cluster and the cable itself and the hole looked way big and I.tried using rtv in there when I installed the cable and locked it in place....no.difference (I might try again).

Next step is for me to check the transmission.side.

Anyone have any advice/tips or suggestions?

Rotary > Pistons

 

I could always hear mine when it was slipping, but you should be able to reach under to where the speedo cable reaches the tranny, and then just try to hand-tighten the nut to see if it might just be slipping.

 

I bought another gauge cluster and I'm going to try using that see if it fixes it

Rotary > Pistons