Rev-fire will always be slower then a radar gun, because it measures average speed, not top speed like a radar gun. The biggest variable here is the rev-fire, since the speed reading can be altered by how close or deep the catcher catches the ball.
-W
Which brings up a good point.
It is very difficult to get the distance correct with a rev fire. Rarely do you catch the pitch at the back of the plate triangle which is the distance marker. Even more challenging to the distance issue is the fact that the pitcher never realses the ball at the edge of the rubber which is the other distance marker. So unless you are measuring from the location your DD releases the ball and then you catch the ball at the proper distance from that point; you are not getting the true average speed. AND unless all PC's and Parents are ensuring the distances are correct you can't compare speeds between pitchers either.