While I'm at the conversion we're thinking about changing to either 3.9s or 4.11s in the diff, what would I be turning at on 100/110km/h?

At the moment I'm doing 2000 RPM in 4th with the auto with 15" wheels and 205/65 tyres, 3.45 in the rear. What would I be doing with the 3.9 or 4.11?

Also its going to knock my speedo out by a huge margin, what can I buy to correct that?