Decoupler query + travel tales

Just drove my freshly acquired 1989 Deluxe Window Van from Camberra to
Brisbane - no problems at all! Fuel economy was brilliant (<10
litres/100) until I realised the odometer (and speedo) was wildly
optimistic. I guess around 10% error. Still gave very good economy
which may indicate healthy motor.
Currently has 185/80/14 tyres which may have accounted for it having
to be tacked into the cross winds the whole way - thinking of going to
205/75/15 as I already have alloy wheels - any comments?
Also would like to purchasr a decoupler - any in Australia for sale?