Hi Dean
I'm new to this chat-line thing and still learning the ropes, but thanks for your comments. You make some good points I had not addressed.
Wheels and tyres are definitely NOT standard issue. Wheels are 16" Momo brand competition types with rims, 7" wide front and 8" on rear. Fitted tyres with rolling radius close to original Mercedes specification. Rear tyres were within +0.2% of original spec and they are currently 1/2 worn, so that wasn't the problem. My speedo error was +10% at 100kph (65mph). This was measured against my Uniden "Guardian Angel" GPS speed minder (designed to act as a speedometer), and very accurate on straight roads.
I had previously checked my odometer against highway markers and it came up spot on for distance, so I assumed it was not a sender problem.

You are 100% right about that resistor. According to the data sheet this resistor is provided to allow the auto manufacturer to compensate for calibration variations in individual speed meters. That would not work if it created serious non-linear effects. My resistor was marked 75.0ohms (an "odd" value), and measured 76.1ohms so it can't be causing the 10% error.

I now have the (adjustable) speedometer reassemble and on the test bench. It comes up as correct and linear all the way from 40kph through to 200kph, and only shows errors of 1-2 kph below 40 (not a problem).
Next step is to finish the cluster overhaul and get back out there on the road. Then I'll know the real truth.

I have no idea why you had such problems with your 300SD conversion. Is it possible the sensors are different? There ARE some components that effect the way the ITT chip "shapes" the sensor pulse-train before it is processed. Two of the resistors appear to provide a custom-trimmed value, and one of these appears to have been installed at Mercedes "assembly" time, not when VDO made the PCB. Can you remember if the other resisters and capacitors next to the chip were identical between the two instruments?

3D printed gear (12 tooth pinion) is fitted and works fine. Pattern now posted on Thingiverse. If the next gear in line lets go it'll be time to contact Pelican Parts, it would much easier than designing another.