ERF-NGC-European:
There’s a chapter on this in Lorries of Arabia 3
Hopefully it goes along the lines of an ex fire truck test driver raised issues outside of these pages concerning ERF’s choice of gearing in the form of direct drive Fuller v over drive version.That seemingly being based on erroneous worries concerning prop shaft speed when the prop would obviously have been turning at the same speed at the equivalent road speed but the engine speed would just have been less.In addition to the closer spread of ratios in hilly terrain using the 13 speed not 9.
The above seemingly confirmed by at least one customer having called for the replacement of the gearbox of a new vehicle prior to delivery exactly in line with that claim. 
