standard axle on a 90 spec saloon is 4.24:1 on the deluxe the diff ration is 3.89:1
other options are magnum axle which could be anything up to 3.45:1 BUT new prop would be needed, new inner trialing arms and a new handbrake cable setup.
perosnally if your undergeared its either swap rations in the type 9, (lots of variations off the shelf) and or larger back wheels