ACT Research said the final number will approach 30,100 units.
FTR Associates said orders were down 22 percent from the same month a year earlier.
Class 8 new and net orders rose to six-month highs in October.
ACT Research said preliminary net orders for October improved both month over month and year over year.
Class 8 net orders rose 55 percent year over year and 12 percent over August.
Preliminary September orders increased 12 percent over August and 56 percent over the same month a year earlier, FTR Associates said.
Research firms forecast August build rates of more than 20,500 units, higher than the July level.
Preliminary orders fell 12 percent in July from June to the lowest monthly total since September 2010.
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