Differentials need to cool off too. We put bigger tires on trucks, haul huge loads, and on top of that, do it with speed. Those poor rear ends are just burning up inside since we're not increasing their cooling capacity when we make all these mods. One great way to keep your rear axle cooler is to use a larger capacity differential cover like this one from Mag-Hytec (818.786.8325, www.mag-hytec.com). Not only does it allow more fluid to be held, it also has a finned aluminum housing that acts as a heat sink, dissipating a considerable amount of heat over your stock differential cover.