Why does my car top out at 120????

My car does the same thing. Atm it is stock except for underdrives and a K&N. I also have a Tremec (Billetsteel flywheel to aluminum D/S, M/T Computer and harness) inplace of the AODE.

A friend and I were just messing around one night on a long strip of road and I was kinda upset that my car only showed me going about 120. The car felt like it was still pulling but the speedo just wasn't showing it. (This was while the aode was in) He mentioned the speed out of the blue and I was like huh? He said yea we were going 135 or so. I thought he was BSing.

When I had the tremec installed with the correct speedo gear, I later took it out to a long stretch of road and tried it again. Same thing. I only saw 118 or so on the speedometer.

The car feels like it is still pulling but doesn't show it on the speedometer and the car feels really light so I start letting off. Maybe itst he speedometer or something, I dunno. I am in the process of lowering my car 1 1/2"- 2 " so one day I'll try it again and see if it improves any. I will agree with the others that I don't really care about top end, I love the 1/4, but found it annoying as well that I could only go 118-120.
 
With the car Lowered now, it doesn't "float" as bad as it did. I pulled my car up to 125 today by my speedometer (which as mentioned earlier I think is wrong in comparison to a friends). The car seemed to continue to pull but I need to get an allignment now and I was running out of road. I think it is a definate improvement but still, I don't get off on top end, I like the 1320'.
 
I topped mine out w/ 2.73 at about 145 but put in the 3.55's and w/ a 26 inch tire your top speed in 4th will be approximately 130 mph. W/ a few mods a 5th gear pull and alot of balls gets you up to 160mph+ :), I did it when racing a '98 cobra, he was mad when I pulled away in 5th gear.