Hello Basil,
Please do a search on this site. There are many good posts on speedo cable replacement. Based on the help of friends and experts on this site, I replaced mine recently. Here are some notes that may help you.
Remove the Tach, Center instrument gage and then the Speedo. Refer to other posts on this site to perform this procedure. Cautions are mentioned in these posts regarding the capillary tube. Please pay attention to these posts and warnings. With the Speedo pulled forward, loosen and remove the speedo cable.
Then loosen and remove the speedo cable on the transmission side. The transmission end of the speedometer cable is held in by a 5mm allen screw which clamps a synch type arrangement..
The 5mm allen bolt is almost directly above the shift rod going into the tranny on the driver's side. Remove the bolt completely and wiggle the cable end until it can be removed. Resisit the urge to stick something into the slot to pry the synch apart otherwise the aluminum casting will break.
To remove the speedo cable, you need to slide it through the firewall and into the car’s interior, whereas the end of the cable which attaches to the speedo is too large to fit through the hole on the firewall. Attach string or wire to the end of the old cable as you pull it through and into the car. This will help you to pull the new speedo cable back through the firewall again.
After pulling the new cable through the firewall, install the transmission end of the cable first. You may have to turn the cable a bit during installation to allow the cable to "key" into the drive.
CAUTION… Whatever you do, don't overtighten the 5mm bolt that holds the cable end to the transmission rear cover. While the attaching area looks like a clamp fitting, it is very easy to fracture it by overtigthening. What holds the cable in is the bolt riding through the groove on the cable end.
Good Luck!
Bob
bpossel (Memphis, TN.)
'71 280SL / '97 E320