A stock manual 5.0 should top out at about 140 and an automatic at about 135. In the case of an automatic with a stock 2.73 rear end, that top speed will be reached in 3rd gear. Overdrive is ridiculously tall and you'd be lucky to reach 125 in that gear even on a long stretch of road. If there's a slight headwind, 115 is probably the limit.
2006 GT - 5MT - 12.8 @ 108 - 306rwhp 324rwtq (E)