Speedometer calibration - TH350 where to begin?

[75stepside]

Ok guys, i finally got my speedometer to read!!! I have a Garmin GPS and so i had it along with me to see how accurate i am... well, i'm way off. My in dash reads 55 mph when i'm doing 35 mph!!! and reads 80 mph when i'm doing 55 mph! so, i need to get it calmed down. Here is the current configuration:

TH350 Tranny
10 Teeth Drive Gear (red or purple; i think its 10 teeth??)
19 Teeth Driven Gear (natural)
3.73 Rear Axle Ratio
27" Rear tire size (295/50/15)

what gears do i need to change? both or just one!?!? i have no idea what my original tire size/rear axle ratio was or if these tranny gears were even installed by the factory or just put in... any direction would be helpful.