I have a long-standing question that perhaps you can answer. For years I've driven a two-wheel-drive pickup truck, not really needing a four-wheel-drive vehicle. However, in the winter, when it does snow, I traditionally throw sandbags in the bed to increase traction. The thing is I don't know really how much weight makes a difference or if it's safe to do so. I realize trucks are designed to carry weight beyond the weight of the vehicle itself much more than a car. But what is the optimal weight to carry? Is there any rule of thumb about how much weight to add to a standard half-ton pickup to increase traction without adversely affecting handling, not to mention gas mileage?
The whole idea behind a pickup truck's architecture is to carry significant amounts of cargo, as opposed to a passenger car which consists of the passenger compartment and room for a couple of suitcases in the trunk. Now, in today's society we see many pickup trucks with unused, empty cargo beds. Why? Your guess is as good as mine, but this is where a traction problem comes about.
Since the truck was designed to carry a payload, an empty cargo bed leaves very little weight on the rear axle, and therefore limited traction at the rear wheels. We were unable to find any manufacturers that give specific instructions in their owner's or service manuals regarding the addition of weight to gain traction. But common sense tells us that it's a good idea to do so during hazardous driving conditions when the wheels are spinning and you're getting nowhere fast. This is especially true when new snow tires and/or chains are extravagant for the situation at hand.
Sandbags work really well, but remember to place them directly above the rear axle. If the additional weight is placed too far behind the axle it may actually "teeter" the truck and lighten the front-end causing handling instability. As far as the amount of weight to use, you're going to have to play it by ear. The best way would be to gradually add weight while road testing the truck for traction on a slick surface. Then, by simply looking in your owner's manual under "standard" and "maximum" vehicle payloads you'll see the safe weight limits to work with without hampering ride and handling characteristics. But you should definitely see a traction improvement before coming close to the standard payload or losing MPG dramatically.