Rpms with 3.73 gear
 

How many rpms turn with 3.73 gears in a 98 gt Aod in last gear (4th) at 75 mph and 90 mph. Does the speedometer change when you change the gear ratio's

You need either a new speedo gear or a compensator to correct the speedometer when you switch gears. As far as how many RPMs, take what you are at now and multiply it by (3.73)/(your current gears)