When I checked the speedometer from my '79 SC against my GPS it seemed to be linear but about 5 mph fast. Others have reported similar results.
I had to pull the speedo apart to replace the odometer gear. While I was in there I thought I'd see if I could calibrate the speedo. I found a thread here that describes how its done but the description was somewhat sparse so I thought I'd describe what I did.
The adjustment is a small adjustible resistor near the back of the mechanism. The problem is that to get to it the housing not only has to be drilled, it is also necessary to go through one of the plugs that the instrument's light goes into. This is very soft material that does not drill so it was necessary to first drill the housing and then grind the light plug by going through the drilled hole with an 1/8" bit on a Dremel tool. Once it was ground I then squeezed it with pliars so that the light will be held snugly.
After reassembling the speedometer I purchased a set of TV set adjustment tools. These are just small screwdrivers that reach about an inch and a half into the speedo. I put the instrument back into the car but left the one light out. After checking the speedo against my GPS I gave the resistor a turn to the right (clockwise). Checking again I discovered that the speedo is now about 1 mph slow. A second small adjustment to the left was all it took for the speedo to be just about right on.
The speedo is now about one to one and a half mph fast at 30 mph or below. I checked at 40, 50, 60, 70, 80 and 90 mph and at all those speeds the speedo is within one mph of agreeing with my GPS. Unlike most instruments the VDO speedometer appears to be very linear but just set a little fast.
Once calibrated the last step is to seal the hole with a spot of RTV silicone and put the light back in.
Here is a photo of the drilled speedometer housing.
