As a vehicle travels down the road, the tires rotate a certain number of times per mile depending on the tire circumference. If all the tires are not exactly the same circumference, they will turn at different rates. On these trucks, 4WD is a “part-time” system designed for use only on low-traction surfaces.
This system has no internal differential, so the front and rear propeller shafts will turn at the same speed. On low-traction surfaces, different tire sizes won’t have much effect, as one tire can easily slip on the road surface to match the speed of the other tires.
On a high-traction surface, it is much more difficult for one tire to slip. Therefore, the 4WD operates under a “blind” to heavy-load condition, and noise is the result.
As tire rolling rates are a major factor in this condition, consider the following important items:
• All tires should be the same size and brand (excluding the spare tire) because tires of different brands may have different circumferences, even if they are the same size. • Tire pressure is equal in all four tires. • All tires have approximately an equal amount of wear. • All tires are the same tread type. Don’t mix on- and off-road, all-season or street tires on the same truck, as they may have different circumferences and roll at different rates.