Well I guess your car somehow defies the laws of physics and mathmatics, or you have something wrong with your speedo.
When you change rear end gears, or the gears for the speedo, you are changing the RATIO. The ratio, or the change in ratio, can be expressed as a PERCENTAGE. The percentage of change made to the rear end gears will change the percentage of error in the speedometer by the same amount. That is basic math. No gear swap or tire height change you can make will change your speedometer's reading by 10 mph throughout it's entire range.
Perhaps you should get your radar gun calibrated along with your speedometer.
The driven gear is an easy, 5 minute swap. The driven gear requires you to take the tailshaft housing off and remove the drive gear from the output shaft.