My 1978 300CD just had a new speedo cable put on, but it appears to about 5% off. If I am driving 70, the speedometer reads about 78. Is there any way to adjust this?

 

POSSIBLY.....

Speedo installations are legally manufactered to better than 10% accuracy. The implied error is skewed such that you are travelling slower than the instrument indicates for liability reasons. So your installation looks as though it is in "spec".

Have you changed the wheels and tyres? i.e. they are no longer the correct OE rolling radius. If you have changed wheel and tyres to a previously OE available but not originally fitted size then there is the possibility that you can purchase a different drive gear at the gearbox end of the cable. However this may mean a gearbox strip.

Incidentally accuracy changes due to tyre wear. New vs old (smaller = high reading).

Another possibility is that the 8mph error is roughly constant throughout the range in which case with good DIY skills you may be able to move the zero position of the speedo needle on its shaft by holding the inductor ring inside the speed whilst twisting the needle on its shaft (taper fit). this MAY give you a better accuracy compromise.

How do you know you are doing 78 rather than 70? How are you measuring distance covered / time taken and what precaustions have you taken to ensure accuracy?
Part of the "error" MAY BE down to your measurement process.

Hope this helps,
Stuart

 

the simple replacement of the cable does not alter readings. It was off prior to the cable replacement