All four tyres had to be of the same make and model, and all four tyres had to be replaced at the same time — if one tyre was damaged or punctured, the three remaining good tyres also had to be replaced. In addition there were other maintenance requirements which were both exacting and unusual. Neglect of these points through ignorance or a misconceived attempt to save money was common, and was likely to lead to very expensive failures of the transfer gearbox.