285 vs 305 vs 295???

[AussieOiler]

G'day Fellas,

I have an Edge Evo with a dash pod monitoring vitals such as EGT, TFT, EOT & Boost.
I'm soon to go for the Upgrade to the Gryphon.

A few questions I'd like to ask your opinions or actual experience (prefered) on the differing effects that tyres sizes have on performance & MPG.
Does programming the Gryphon custom tunned for the tyres size have any real noticable advantage to a Gryphon canned tune?

I am currently running Toyo OPAT AT 305 70R 16s (33.2' X 12.0") on 10" Rims.
I have a Stock Tranny (extra coolers, thinking of a JWVB) and 3.73 gears.
Questions:
1. Would there be any real advantage in dropping back to a 285 75R 16 tyre? I realise that there is a neglegable difference in the overall diameter between a 305 & a 285. But the difference in rolling resistance is to be taken into account. That is there is 80mm (3.2") ( 4 x 20mm) more rubber contact with the road surface to overcome. Plus I guess the extra weight of that extra rubber and the 10" Rim compared to an 8" Rim for the 285 size tyre.

2. The reason I've thrown a 295 75R 16 into the equasion is that if you blokes come back with good reasons/advantages for dropping from a 305 to a 285 I would also have to get 8" Rims as the 10" are too wide for a 285 tyre.
However a 295 tyre is around 33.4" in diameter (1/5" taller than the 305/285), but with less tread width (rolling resistance) and would fit a 10" Rim (just).

The truck is quite heavy even with an empty load. With all the extras, steel bull bar, bumper, canopy, long range tanks etc etc; it weighs around 4,200kg (9,200lbs).
Now fully loaded with all the 'goin' bush gear' and towing a Rough Road HD suspension caravan the GCVM (GCVW) would come to 7,100kg (16,300lbs)

So am I looking too deeply into this? (or) Is there some real gains too be made with either tyre size changes or Gryphon programming or combination of both

I'm very new to all this, so go easy on me fellas, living downunder in Aussie standing upsidedown on the bottom of the world tends to let too much blood run to our heads

Cheers fellas,
Reg.