"... trucks with properly maintained brakes generally take 25 to 65
percent longer to stop than a car or, stated differently, they have a
60 to 80 percent braking efficiency. The greater efficiency would be
for a fully-loaded truck and the lesser efficiency would be for an
unloaded truck. These increases in stopping distance are partially
attributed to the tires used on trucks. Truck tires are made from
very hard rubber, so they will last much longer than car tires.
However, this feature causes them to generate lower friction values
than the softer-compound car tires."
If true, it's interesting that a loaded truck can have better
stopping ability than an unloaded truck. Reading between the lines,
I guess the implication is that this is the result of the hard tires
better molding to surface irregularities in the road in the loaded
condition, increasing the effective coefficient of friction.