My '88 260E speedometer seems to be pretty accurate. It rests at zero. In fact, it doesn't even get off the zero until I'm going 5 or 10 mph.
The 15 mph error is a lot. I don't know if this is possible but it sounds like the pointer needs to be pulled off and reset to zero, or better yet, find a zero adjust screw somewhere on the speedo if there is one. Maybe someone else here will be able to help.