Hey Joe, thanks for pointing this out. I ran the numbers myself and i know what you are talking about. I believe my tach is very good, i don't think that is the issue.
There are two factors that change your calculations...
1)Rolling Circumference....i can guarantee that my tires are not 26" in diameter anymore Hahaha..im sure i've burned off at least a 1/4" on both sides...so i'd figure 25.5" diameter. But running the calculations with the circumference associated with a 25.5" diameter would put me at 119.944mph @ 6500rpm or 118.099mph @ 6400rpm...so why was i only doing approx 113mph at 6400-6500rpm crossing the lights before the nitrous? maybe because...
2) Slippage of the torque converter? If i was supposed to be doing 119.944mph @ 6500rpm and i was only doing say 113mph then the slippage in the torque converter is 5.79%. If i was supposed to be doing 118.099mph @6400rpm and i was only doing say 113mph then the slippage in the torque converter is 4.3%.
These percentages seem kind of high though. Shouldn't i only lose 1-2% once the converter is "locked up"?
well any way i look at it....i think im going to end up trying a 28" diameter tire, i just need to make sure it will fit, based on the width of my 9" rear end, backspacing and offset of my rims and spacers and the inside of my wheel wells