I agree with barna,
I would install the 2.75" Beaudesert Exhaust on the 3ltr D22 Navara. The guy has been in the industry for many years and they also test all there exhausts for the best results instead of just going with what they 'think' could be the best size. In saying that, the 2.75" would be best suited to a Factory Engine; meaning that you dont have any major internal modifications to your motor it will offer you the best overall performance from you truck mate. I have the full 2.75" system from beauy exhausts where they replace the dumppipe off the rear housing of the turbo and replace the rest of the system with a straight through system and added a straight-through-muffler. But I have also driven a mates truck that had a 2.5", there was no major difference except I had more top end power out of my system.
I think that a 3" system would be a touch more laggy cause of the added diameter and would be alot more sluggy off the mark. You have to keep in
mind, sure the 3ltr patrols have 3" exhausts but they have a slightly different ZD30 motor to what is in the Navara which is why they get away with it added diameter.
It was a mod definitely worth doing but just make sure you know what you want to achieve when selecting the size.
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Beaudesert Exhausts <-
Beauy Exhausts also post their ready-to-go Exhausts systems all around Australia. Which that should tell you enough that they sell enough of them to have them in stock ready to bolt-on.