The last time around, we added a larger, slightly heavier wheel and tire combo and lost a little more than 1 mpg. Granted, with the 6.7L's whacked-out emissions system that dumps raw fuel into the engine on the exhaust stroke to burn the soot from the DPF, who's to say what the real mileage numbers could be? But despite the capricious and fuel-wasting antics of our vehicle's active regenerations, our test data is what it is.
One of the things we've never been too fond of in Cummins-powered 4x4 Dodge trucks since '94 is the absence of locking hubs on the front axle. Worse still, is the fact that the factory wheel bearing assemblies place the wheel bearings very close together, increasing leverage loads and hastening bearing wear if heavy hauling or big tires are involved.
Dynatrac's Free-Spin kit addresses these shortcomings with an exquisitely machined spindle that bolts to the factory '94 to '02 Dana 60 or '03 to '09 AAM steering knuckle to accommodate a 1 -inch 35-spline stub shaft, hub assembly, and locking hub dial for true two-wheel-drive operation. In addition to the added strength of the 35-spline stub shaft, the Dynatrac hubs space the inner and outer wheel bearings farther apart, decreasing the effects of leverage to increase bearing life. Furthermore, the wheel bearings in the Free-Spin kit are serviceable. Unlike the factory unit bearings, you can pull the Free-Spin bearings out to inspect, clean, and repack them with grease, as well as adjust the bearing preload. They're long-lived units that will take all the punishment these trucks can throw at them.
Although it's a relatively straightforward installation that most guys with moderate wrenching skills can handle, we took the easy route and headed down to TAG Motorsports in Escondido, California, to have our Free-Spin kit installed by shop foreman, Jay Miller. Miller and the rest of the gang at TAG have a lot of experience in wrenching on diesel trucks and had our Dodge ready to roll without any hiccups.
With the kit installed, we were pretty surprised to find a noticeable difference in steering feel. Given that unlocking the hubs stops the axleshafts, ring and pinion, front driveshaft, and front transfer case output shaft from spinning, it stands to reason that omitting all that rotational mass is bound to have a positive effect on noise, vibration, and harshness (NVH) quality. And as you can see, the elimination in drag resulted in a very definite increase in fuel economy. Come on by next time when we'll be tossing a Fitch Fuel Catalyst into the equation.
 TAG Motorsports' Jay Miller...  TAG Motorsports' Jay Miller began the install by removing our Dodge's brake calipers, rotors, and caliper brackets before removing the bolts that hold the unit bearing and axleshaft assembly to the knuckle. Then the unit bearings were separated from the stub shaft and the U-joint caps driven out to separate the stub shaft and inner shaft. |  Unlike the older Dana 60 front...  Unlike the older Dana 60 front axles, the AAM front axle in '03-and-up trucks features a funky-sized U-joint that requires these steel shims to be inserted behind the U-joint cups. Miller laid the shims in place and then pressed in the U-joint caps to install the new Dynatrac 35-spline stub shafts on the factory inner shafts. |  With a careful touch to avoid...  With a careful touch to avoid damaging the inner axle seals, Miller slid the shaft assemblies back into the housing. The Dynatrac instructions are very detailed and walk you through every step of the conversion if you're a do-it-yourself guy. |