Here's a section at 25 frames /sec , the speed the camera would need to have been running at to get McLain up to BB&N's 17 mph at the Main/Houston junction :
See movie section at 16 frames/sec
Note the prominent guy in the black suit walking left to right. (Yeah, that's just gotta be Jack Ruby...) Watch
out for a kid who runs right to left, in front of him, at positively super-olympic speed, here. :-)
"The audio record shows the sound of the motorcycle motor at a constant level until 3 sec before the first putative shot, when the motor noise drops to one fourth of that level [8 HSCA 11]. This requires the motorcycle to travel at a faster speed prior to the shots than during or after the shots. By setting H-20 at 6-7 sec prior to the first shot a trajectory of about 20 mph is projected for McLain's motorcycle on Houston Street. This speed is about twice the speed of the motorcade itself and is required because at this time, the President's limousine has already turned on to Elm Street, 220 ft away." ( Thomas, 'Hear No Evil' ].
Note that the 'bike engine speed' is constant from before the Main/Houston corner to just before the Houston / Elm corner. Thus McLain's speed ( as seen in the Hughes film) is also his speed travelling up Houston. If Thomas is right about Mclain's speed being 20 mph along Houston, then the camera has to be running at about 27 frames /sec - about 50% faster than it was designed to run. A glance at the 25 frame/sec movie (above) will show that this is just not realistic. Mclain was actually doing about 11 mph at Main/Houston, and ( according to the DPD tape itself ) cannot have speeded up after the bend, but carried on at that speed till before Elm , where he actually slowed down. ( On the unwarranted assumption that the tape is anything to do with McLain )
The graph below shows the relation between the assumed frame rate of the Hughes movie & the resulting speed
calculation for Mclain at Main / Houston. ( And thus for his speed along Houston). The (very reasonable) 16 f/s
value gives about 11 mph. To get up to BB&N's 17 mph , we would need a frame rate of about 24 f/s. To get to
Thomas's 20mph, about 28 f/s, which is pushing double the maximum theoretical speed of the camera ( 18 f/s).
At Paul Hoch's request, here's a graph showing the speed McLain needs to attain versus the Z frame # taken to match
H20 :
Obviously the Z frame numbers below 133 are notional. The grey verticals mark one second intervals, either side
of Z133.