[martin_r_smith]

Regarding gear ratios, its an M56 gearbox so im sure there are options. Personally not looked into it as I dont really care about economy.

Bigger wheels would help you yes, not sure aboute the speedo part, but to be honest it probably wont be that far out as what you gain in wheel you lose in tyre profile.

Also got to remember this is quite a revvy engine for a diesel, they care not for being driven off boost (below 2000rpm).