I’ve read what seem to be conflicting accounts of Bill Greer’s testimony...
Did he think gunshots were a motorcycle backfire... or a blown tire... or, whatever?
The “blown tire” idea has been used to explain why Greer might have slowed (or even stopped) the vehicle... which seems to be corroborated by numbers who testify to this... (and perhaps the quick advance of the motorcycles beside the limo in Zapruder confirms this)
Greer also said he accelerated simultaneously to Kellerman telling him... and this is conflicting as well... “Let’s get out of line, we’ve been hit...” or “let’s get out of here, we are hit...”
Watching Zapruder, it is obvious that Greer accelerates only after he sees that the president’s head has been clearly hit...
