As has already been said MPH at the 1/4 mile dyno is a direct translation to HP. There are no variables. There are variables when it comes to E/T (traction, diff gears, tyre size, convertor etc. etc.) but MPH will always be the same.
For arguements sake, i went 13.8 (might have been 14.0, been a long time) with my old blown V6 in my car on street tyres, then bolted slicks on and went 13.4. However, both runs were at the same 102MPH. This is because the power was the same. The only thing that had changed was the traction. So, i got over the line at the same speed, but i got to the line faster. That's how the drag strip works.
Also, reaction time has nothing to do with either E/T or MPH, it is simply the amount of time between the green light dropping and the car breaking the starting beams.
Dynos are easily fudged and as such are nothing more than a tuning tool (as Cava said). MPH can never be fudged.