Ryder System Inc., the largest U.S. truck-leasing company, said its third-quarter earnings were less than it forecast because a weakening economy hurt its commercial- truck business. The shares fell as much as 8.1 percent in early trading. Ryder reduced its earnings-per-share estimate to a range of $1.12 to $1.14 from $1.20 to $1.23, according to a statement today. ``The announcement is related to the drop in commercial rental market demand,'' said David Bruce, a spokesman for Miami- based Ryder, in an interview. ``We've also had to sell our used commercial trucks wholesale to reduce our fleet'' of about 160,000 vehicles. Ryder said it expected ``softer economic conditions to continue'' into the fourth quarter. The company dropped its full- year estimate to a range of $4.10 to $4.15, from $4.30 to $4.35. First sign of softer economy ?...