Take Manchester Expressway (Ga. 85) north through Manchester.
In Woodbury, turn right onto Ga. 18 and follow it to Zebulon.
Turn left onto U.S. 19. It merges with U.S. 41 (left turn) a couple miles south of Griffin.
From here, youre about 13 miles from the entrance to Atlanta Motor Speedway, which will be on your left.
I've made this trip in a little over 90 minutes (don't be stupid driving through towns) and unless everyone takes my advice, you should only run into heavy race traffic once you reach Griffin.