GEAR VENDORS gives your vehicle up to twice the forward gear ratios for unparalleled performance. Most auxiliary transmissions are just dual ranges (put them in low for towing, high for running empty) but you still have the big gaps from one gear to the next that rob performance. You could actually go slower than you do now. The UNDER/OVERDRIVE™ is built for giving you a progression of gears with 1/2 the drop in rpm of the stock vehicle so that you stay in the power with close ratio performance. More power than just an underdrive or overdrive and just one of the reasons the GEAR VENDORS UNDER/OVERDRIVE™ is the most industry awarded transmission you can buy.