I also run car tyres on our TD. Ours weighs around 620kg and I have a No.9 Dexter torflex axle. The axle we bought was a 5 stud so we went with 14" rims as they are easier to find in 5 stud than 13". Also amongst the stack of tyres I have stashed in the back of the shed, left from when I sold my tyre shop a few years ago, I have a bigger selection of 14".
Considering the total weight of these things, the car tyres have ample load rating for the job. I am running 195/60R14 with 35 to 38 psi, which is just under their max . The over all diameter on those is less than the 185/80R13 that you are using. Also if you keep the pressures up, you will havea lot less tendance for swaying, and that goes for the rear tyres on the tow vehicle as well. The thing is, it is the springs that should be determining the ride quality, not the tyres.
hwitten wrote: Will also do a self imposed speed limit of 90/55 Kmh/mph when traffic allows. Not always possible on 2 lane with semi at back door but will try.
Here in New Zealand the speed limit for all heavy vehicles (gvm over 3500kg ) and all vehicles towing a trailer (regardless of size) is 90 kph.
