The 350 Lockup transmission uses unique hydraulic circuitry to apply a clutch in the torque converter to completely eliminate slippage. What this does for the enthusiast is allows higher stall speeds to be used for acceleration without causing slippage and excessive heat build up when cruising just like the overdrive GM transmissions use. The lockup feature will typically lower cruise rpm by 200-500 rpm or even more depending on speed, stall speed and rear end ratio. This translates into improved fuel economy and lower operating temperature of both the transmission and engine. This is a great alternative to an overdrive transmission when space is a concern and is considerably less expensive as well.