* Posts Q2 EPS of $0.65, below $0.69 Wall Street forecast
* Profit margins lower than expected
* Lowers forecast for heavy truck sales
* Shares fall 10 percent (Updates stock action, adds analyst comment)
BOSTON, July 26 (Reuters) - U.S. heavy truck maker Paccar Inc (PCAR.O) missed Wall Street's profit forecast on Tuesday, sending its shares down 10 percent, their biggest drop since late 2008.
The company, which makes trucks under the Peterbilt, Kenworth and DAF names, also lowered its forecast for overall tractor trailer demand this year, citing the uncertain economy.
The company said profit more than doubled to $239.7 million, or 65 cents per share, compared with prior-year earnings of $99.6 million, or 27 cents per share.
Analysts, on average, had expected profit of 69 cents per share, excluding one-time items, according to Thomson Reuters I/B/E/S.
Revenue rose 61 percent to $3.96 billion.
"I don't think the profit margins are where people expected them to be," Robert W. Baird & Co analyst David Leiker said.
The Bellevue, Washington-based company expects makers will sell between 180,000 and 200,000 Class 8 trucks -- the heaviest vehicles including tractor-trailers -- in the United States and Canada this year. That is down from its April forecast, which predicted sales of 200,000 to 220,000 such trucks, which would have been the industry's best year since 2006.
"We have lowered the range due to the uneven economic recovery and supplier capacity constraints, especially tires and chassis components," said Dan Sobic, an executive vice president with the company.
Paccar shares fell $4.90 to $45.50 on the Nasdaq, briefly touching $44.25, their lowest point since September. (Reporting by Scott Malone, editing by Gerald E. McCormick and Derek Caney)