Allison Transmission Holdings Inc. reported a decline in profit for its third quarter, hurt mainly by lower revenues.

Net income for the quarter declined to $32.2 million, or 17 cents per share, from $38.8 million, or 21 cents per share, in the same period last year.

Revenues declined 14% to $493.5 million, down from $574 million last year.

Gross profit for the quarter was $224 million, a decrease of 13% over gross profit of $258 million for the same period in 2011. Gross margin for the quarter was 45.5%, an increase of 44.9% for the same period in 2011. The increase in gross margin was principally driven by improved manufacturing performance, favorable material costs and price increases on certain products.

The Indianapolis-based company attributed weaker sales to its North American off-highway sales, which fell 71%. The drop largely happened because the company experienced unusually high demand a year earlier as natural gas fracturing went through a boom, Lawrence Dewey, the company’s president, CEO and chairman, told investors.

Allison’s biggest market, North American on-highway sales (commercial semis), fell 5%, to $189 million.