Speedo adjustment

[Beer4Blood]

04 GTO Ls1 - so I adjusted the tire size and then the axle ratio in my t1000. I just swapped to a 3.70 gear set and was hoping to rectify the speedometer situation. The closest option available in the tuner was 3.73 not a big deal should def be closer than I am currently. WEEEELLLL ..... after uploading the parameters 275/40/17r and the 3.73 gears the results were hilarious. It adjusted my speed alright but in the totally wrong direction. I was going about 40 on my GPS and my speedo was registering 100 so what's the fix here?