Tansis, you mentioned Tachographs, I used to work for a place that built the division/multiply ratio boxes that were installed with the old VDO analog "circle chart recorder" tachographs. There were lots of tamper seals on the boxes, and a really lame key on the tacho itself, but the main deterent to tampering was simple.... broken seal, no payment.
The sender was mostly a mag sender on the drive shaft, impossible to alter, and the division/multiply box was right beside the tacho, in plain sight.
Not many were ever tampered with at the risk of no payment for a weeks haulage.