Speedometer adjustment
 

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?

5% would be 73.5 mph, you are at ~10.3% off.

You might check and see if the tires and rims are stock or if they have been changed in the last 28 years?

Speedos in that era are typically problematic. My 85 wobbles so much I usually couldnt tell you exactly how fast I am going.

A good VDO repair shop can calibrate that speedo, but why spend $100+ ?? for such a small % problem?

Think of the savings!
 

Think of how much you'll save in speeding tickets. If you drive like most of us do, you'll always be pushing the limits anyway. This way you have a margin of safety. See, there's a silver lining in every cloud.

Anything else I can do for you?:D