Right. You really ought to do much more research.
And I hope you take this the right way, but if you want cruising from a Defender. You might as well chop it in now and get yourself a Disco or Range Rover.
Defenders are awesome vehicles. But they will never be quiet refined road vehicles. No matter how much money you waste on one. All the refinement and road manner issues with a Defender are solved with a Disco 1. Which under the body is almost identical to a Defender, but the rest is constructed in a completely different way.
Also all 300Tdi engines are identical pretty much. Be it Disco or Defender. Although the 300Tdi auto in the RRC and D1 uses a different electronic injector pump. But does nothing for refinement or cruising.
What you are probably thinking of is the taller 1.2:1 ratio transfer box fitted to the Discovery vs the 1.4:1 for the Defender. But what people fail to realise is, a Discovery is also fitted with smaller wheels as standard. This means mph per 1000rpm in top gear is almost identical on both vehicles.
You can fit the Disco 1.2:1 transfer box to a Defender. But tbh a stock Tdi probably won't pull the gearing very well and you'll end up in 4th gear a heck of a lot more often. And while you'll get a drop in rpm while cruising. It really isn't going to transform the vehicle.
