As far as the reverse lights have you checked if there is a connection to the gearbox?
that was the problem we had on the cefiro when we got it converted. there was no actual connect to the box.
Im assuming ur speedo is a similar problem with the cabling.
The gears running out fast is definitly the diff.
also, if you notice a higher comsumption of fuel its due to the ECU and diff too.