HPI 39T High Speed Idler Gear... Increase of up to 20mph!?

[JThiessen]

I dont have time right now to look at those vids, but I have some reservations about it. An idler gear is just a gear that sits in between two other gears. In a tranny, its basically "locked" or sandwiched in between the input shaft and the output shaft, thus you can't change the tooth count on it (that would require one of the other shafts to move since a change in tooth count equates to a change in diameter).