Careful with a D2 transfer box without a tune if fuel economy is your goal. I have found the opposite with consumption going up. It seems the vehicle has to work that much harder to achieve the same speed, even if the revs are lower.
I used to be able to get 480-500miles out of my 110 tank consistently with motorway driving, now its a lot more like 390-420.

However, for me, the drivability and quieter motorway driving outweighs the fuel penalty. I would imagine a tuned engine would 'pull' the box better, but that might be open to debate. Next on my list, would have done it a long time ago if it didn't ramp up insurance premiums so much !

A stage one remap shouldn’t affect your insurance premium (“engine chip 10-25%”)
And it makes a big difference to the Defender with no downsides at all.

Try Dynachip. He will come out to you and he charges about £250

I find that l don’t need to change gear so much and it climbs hills in fifth that previously needed fourth.
I would imagine you’d get even more benefit when a Discovery transfer box is fitted.
 
Defender 90 TD5 SW. Last fill up took almost exactly 10 gallons till the pump cut out and again after squeezing some more in. Trip was reading 330 miles since last fill up from same pump - so 33 mpg. The only mods are EGR delete and centre exhaust box removed. The cat is still present. Quite surprised at the figure as it was mainly short trips with no motorway driving, although I never exceed 50 mph.
 
We achieved 32mpg when touring Spain in our 110 TD5
Due to the low speed limits and also 30mph limits out of roundabouts stopping you accelerating fast.
In the UK l get around 26mpg.
 

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