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Just out of curiosity how easy would it be to modify the gearbox on my 1.9 D8 to give it slightly longer ratios in 4th and 5th?

I'm thinking purely for economy not top speed.

Equivalently: will a box from a HDi 90 fit? (5th is definitely longer on the HDi 90; not sure about 4th.) 5th on the HDi 110 is longer again.

Might be easier / cheaper (& more reversible) to change the wheel/tyre combination.

For example, swapping from 195/65 to 195/70 tyres would increase the overall gearing by about 2.8%, reducing engine revs by nearly 100 RPM at 70 mph.

Next option is swapping to an ML5 'box from a 110/136 HDI, (or maybe a 2.1)? which looks tricky to say the least, (I'm guessing your 1.9 will already have the 'tallest' gearing avialable for a BE3/4 gearbox).

I'm already running it on 16" wheels but 205 55 16's, I think the lower profile makes them pretty much the same as the standard 15's in that respect.
Pushing the profile up might make it a bit wallowy in the bends?
Be interesting to know if anyone is running larger profile tyres and how they affect handling as that is certainly an easy option, usually it's bigger rims with smaller profiles - big tyres and a strut brace !!!

I'm just wondering out loud really, probably need a clutch kit in the near future and the bottom mount could do with changing so I thought it might be an opportunity to whip the box out.