What diff ratio will I need?

The Diff.

To get the fuel economy i expect with the V6 conversion the diff ratio will have to change.  I haven't checked what my current ratio is but I'm guessing it is above 4.1:1.  Trekka has 31 x 10's on all four corners which is a bit bigger than what the VT commodore came out with 205/65 r15's, so the Trekka has 31 inch tyres and the commodore 25.5 inch tyres. (According to these calculators differentials.com  and tyre size conversion.com)   

So that is a difference of 5 1/2 inches or 21.5% (don't just trust any online calculator I tried a few and when I worked the numbers back, eg 25.5 + 21.5% = 30.98, they were off. I ended up using this one, csgnetwork percent change calc )

So as a VT has a 3.08 ratio diff and given there is a 21.5% increase in tyre diameter does that mean I just add 21,5% to 3.08?  Giving me 3.7422.  Well I bloody hope so, as that means I can use a 3.727:1, 3.909, or a 4.1 Hilux LSD differential, .  Diff sizes sourced from Drive train gearing bible

After using csgnetwork  Rev calculator I think a 4.1 diff will be the to go.

Commodore V6: 205/65R15 tyres, 25.5 inch diameter, 3.08 diff, Auto 4th .0696
= 1836 rpm@104kph (65mph)

Trekka V6: 31 inch tyres, 3.727 diff
= 1828 rpm @104kph (65mph)

Trekka V6: 31 inch tyres, 3.909 diff
= 1912 rpm @104kph (65mph)

Trekka V6: 31 inch tyres, 4.1 diff
= 2010 rpm @104kph (65mph)
= 1856 rpm @96kph (60 mph)

Now i just gotta find out exactly what diff I have in the Trekka, it sorta looks like a Hilux differential, but not quite.