Better Fuel Economy through Re-Gearing

This is a slightly touchy subject, depending on which camp you're in... Bigger tyres, on the highway, WILL give better fuel economy at highway speeds due to the bigger rolling diameter... around town you may (or obviously have...) noticed a slight reduction in economy, due to the need for more power to turn the bigger wheels from stop/start driving... Personally, I run 35's on my GQ and I barely noticed a difference in my economy around town, but highways I can certainly see the increase (easily .5km/L possibly more...) so for me, I'm not going to bother changing ratios, as the effect on highway driving will outweigh the increase around town... By the sounds though (you say you haven't used 5th for a while...) your engine power is probably not enough to actually benefit from the larger diameter, in which case, lower gears would benefit your vehicle, for both increased economy and better torque around town, and a slower crawl speed in low-range (which is the only benefit I would get in mine... however the TD42 has enough torque that I can pretty much idle everywhere anyway, and don't really need the ultra-slow ratios...). You must bear in mind, that lowering your ratios, will actually increase the engine revs at a given speed...
Oh, and gotta ask... Where can you get your diffs re-geared for $1500...? GQ's are $1200 per end, and that's just for the actual gears, not installation... I would have thought GU's would be similar if not more...? I need a better parts shop... 🙁 hehe...

As far as I'm aware it should, it should bring your revs closer to stock so the motor won't have to run as hard pushing the bigger rubber etc

Args, mate, not arguing with ya or bein smart, but lowering diff ratios, actually brings the SPEEDO back to factory... ie: if for example (in my GQ) the speedo now reads 90 and the tacho says 2500, (assuming this is with the larger diameter tyres... the ACTUAL speed with the factory diff gears will be close to 100km/hr...)... now, lower the diff ratios and the speedo will STILL read 90km/hr and 2500rpm, but the ACTUAL speed will be back to 90... so to do 100km/hr, the revs will actually INCREASE to say, 2900rpm with the speedo now reading 100km/hr... However, the vehicle will actually have more torque at the rear wheels due to the lower gears, meaning it isn't working as hard to maintain the speed... It's a tricky subject/scenario because to LOWER diff gears, the number is higher (ie. 4.6:1 is LOWER than 4.11:1), and while the revs increase, so does the torque, and that actually helps the engine turn the wheels easier... Drag racers will attest to the difference a diff ratio can make... And it can make ALL the difference... Not sure I've explained it properly, but hopefully you can understand it a little better...

I have recently bought a Pajero 2800 1996 diesel I notice that I often am looking for another gear change from 5th up but not available , The revs at 110 kl limit Tas or even when at 100 are about 2600~3000 and I can see a benefit in economy and relieving the engine by keeping on hgw speed at lower revs that the engine would feel happier and my fuel economy, with and on fairly flat highway sections I can see no reason to have revs over say 2300. But naturally the gear ratios disallow this outcome
Any suggestions / comment ( current tyres Pirelli Scorpion Atr - 275/65 R17)