Keeping Up With Today'S Highway Traffic Can Be Quite A Job For Your Engine. Constant High Rpm'S Reduce Gas Milage And Engine Life. Switching To A 3.88 To 1 Ring And Pinion Can Eliminate These Problems By Reducing Engine Rpm At Cruising Speed. Fits Full Synchro Type 1 Transmissions With Keyed Style Fourth Gear And Rhino Transmission Case Above.