30th clone

I went to the track last night hoping for better times out of my 98 trans am 6 speed.. The car used to run on motor low 12.20s on motor at 115 mph an 11.00 128 mph on spray. That was with a 10 bolt with 3.73 an on a 275/50/15 m/t drag radials..I now have a 9in with 3.89 with a spool an 35 spline axles an was talked in to a set of 28/12.5/15 M&H cheater slicks..The car last night would only run 12.54 at 111.00 mph an 11.20 on spray at 122 mph ????
are the tires to big does any one have a idea about what my gear is with this setup ??? I am thinking I am to tall geared now..I beleave I need to go back to a 26in tall tire..
The car has a 224 comp cam,headers,ls6 intake,an a custom tune,150 shot nos.. Please if anyone can Help ..Thank you