In every single case it is important to find the specification of your exact vehicle before making any decision on tyres. Many posts rate the FG rear axle at 4.3T however this is not always the case and threw me off the trail when first visiting this forum until I did the leg work to find the specs for
my exact truck and talking in generalities as if all FGs are the same does not help people at all, it just restricts options for those who don't have the same choice limitations your truck has. This wasn't hard to do, everything you need will be on the drivers and passenger doorframe with confirmation via Mitsubishi/Fuso spec sheets if you want the warm and fuzzy of a second opinion, or to confirm details not on the plates.
The first is the model plate which is useful to check against other specifications for your exact truck. Mine was the frontmost plate on the front passenger door frame. In my case it clearly states it is a FG637EW6V (apologies if the image is on its side for anyone;
The next was the compliance plate which has the correct GVM for your truck. This was next to the model plate above on mine and shows my total GVM to be 5500kg (not 6T as with some other models);
Next was the tyre placard which was on the drivers door frame below the striker plate which says 2 important things; The rear axle rating is 3300kg (not 4.3T), front is 2360kg and that
tyres must not be rated less than 'L' - 120KPH, 'L' is the minimum speed rating (so Michilen XZLs rated to do 110kph are not a legal tyre for my truck);
So this means to run with singles all round for me I need rims and tyres to be rated to 3300/2=1650kg minimum. Googling for "load rating table" for me had the first hit as Bob Jane's web (
http://www.bobjane.com.au/load-speed-ratings.html) which along with every other one I've seen shows 1650kg = a load rating number of '125', so I am looking for tyres rated 125L or better.
As the wheels I am running (Alan's blings) are rated 1800kg they easily cover the 1650kg requirement [hoping this qualifies as this post not being a hijack

, apologies Alan if otherwise].
The tyres are a little more confusing as there are 2 numbers on them. For instance my tyres are 315/75/R16 Goodyear Duratracs and are rated 127/124Q. The first number (127) is the rating when used as a single, the second (124) is the rating when used as a dually with 4 on the axle and the 'Q' is the speed rating. So as a single 127Q is more than adequate for the 125L rating I need and this tyre and wheel is a perfectly valid combination for
my truck
It was also handy (for piece of mind) to confirm this via the Mitsu spec sheet for my exact model and on finding it nothing contradicts the above. Note in the image below the model (exactly with all numbers and letters matching) is listed at the top left and the REAR AXLE section states "Capacity - Axle: 3300kg" and "Load Rating - ADR 24: 3300kg". Under wheels and tyres, whilst the load info was for the duallys, the speed rating at the end shows 'L'. To find this sheet I just had to do some googling etc but everything you need should be on the truck so this is not required, just reassuring.
So my advice to people is not to listen to any advice about this for your truck that is not tailored for it and if in doubt ask an engineer or your state's transport dept, not a salesperson with a vested interest in telling you that what they sell is the only legal option.
You'll also notice that although mine is the crewcab, under the models on this sheet is a couple of single cabs with exactly the same load specs so it may well be that your truck does not require a 4.3T rear rated setup and you can enjoy a much wider choice of rims and tyres than you think.
Just another point on towing discussion, this is in the spec sheet too, notice beside all of the covered models says GCM 6500kg off road and 7500kg on road. This is the gross combined mass of truck and anything towed. It's a good idea to check this for your truck also as going over these limits may also invalidate insurance claims.
Hope this helps,
-Andy