I dont under stand why there isnt more single drive tag axle trucks here. Now with computers and air bags the truck could regulate the weight on the drive axle for better traction.Doug it's the same max GCW irrespective of whether the tractor is a 6x2 or a 6x4 - 44 tonnes or 97k pounds when a 3-axle tractor is coupled to a 3-axle trailer.. Max axle weights are 8.5 tonnes for a non-driven axle or 10.5 tonnes for a driven axle. That being the case the 6x4 has a little more flexibility regarding individual axle weights but that will still not affect the total GCW.
Fuel consumption-wise 6x2 are apparently somewhere between 2.5-3.5% more economical on fuel than their 6x4 brothers all other things being equal. These days it's common to see a lift-up axle on the tractor and maybe 2 axles on the trailer, so a lot of difference in tyre wear when running empty.