It just occurred to me - what exactly is 'ramming speed'? Something less than full speed? More than maximum? Or just fast enough so that when ramming does happen as little damage is done as possible?

But then also something occurred to me some time ago. How accurate can an odometer on a vehicle with ablative wheels be? Say the tread is 10mm thick (not uncommon) - how can it be calibrated for the size of the tyre all of the time? Considering that the 10mm would over time drop to the legal minimum in the UK (1.5mm if I remember correctly) , that would make a difference 17mm x pi = 53.4mm in circumference. Say that a tyre is about a metre in diameter (no I'm not going to measure the wheel on the van) that gives about 3.14m for each rotation. That means for 320 rotations (about 1km) that would give an error of about 17 metres. Not particularly accurate. I wonder if anyone has thought of this? Perhaps this is where the 10% speed 'allowance' comes in (which may be a myth)? Would this also explain the errors I have seen between speedometer values and values measured by a GPS reciever - typically between 3% and 10% depending. GPS accuracy is supposed to be about 0.05m/s, about a tenth of a km/h. Perhaps more on this later.