+1 on RussF post IRT gear diff feel. The Spool is posi-trac and initially will offer more push entering corners off power (requiring suspension tuning to get back entry turn in). On power the Spool offers more acceleration. I prefer the Spool for carpet and asphalt, but it's all driver preference. It did take a few heat races to tune car and get used to the Spool vs. Splipper or Gear Diff up front.