Yes, Bob is quite correct.
The simple formula actually defines a mile as only 5,277.88 feet. For my formula to be true, your lines would been to be 60' - .02414997" in length.
So a Fox Speed model that clocks 36.00 seconds would be scored at 100 MPH my way or 99.96 MPH Bob's way.
Considering the measurement with a tape measure, the lack of a pylon, and timing with a hand-operated stop watch, the 3600 is more than accurate enough.
There are accepted scientific principles against the production of results to finer resolution than the input data.