The 4L60E auto is a 4 speed overdrive with 0.70 4th gear. Depending on your tire size you'd be taching 2700-3100 at 70 mph.
4L60E gear ratios: 1st 3.06, 2nd 1.75, 3rd 1.00, 4th 0.70
I'm running a 3.7 rear gear and 205/50x17 tires, taching 2450 at 70 mph. My old 3.54 tach3 2350 at 70 MPH.
Use this calculator: http://www.catherineandken.co.uk/sti/tyres.html