General Motors Co's U.S. Chevrolet car sales in the first half of the year are set to reach their highest share for the brand in 20 years, outselling trucks in June for the third straight month.

Chevy cars accounted for just above 50 percent of the brand's total U.S. sales for June -- the third consecutive month in the majority -- and 47 percent for the first six months of 2011, said Alan Batey, vice president of sales and service for Chevrolet. He declined to disclose GM's expected total results for June, which the company will report on Friday.

You'll see continued strength in passenger cars, particularly in the compact and mid-car segments, Batey said.

Citing gasoline prices that are still about $1 a gallon higher than a year ago, GM pointed out that 46 percent of Chevy's retail buyers in the first six months chose a more fuel-efficient four-cylinder engine in their vehicles, double the rate of five years ago.

We have been a very, very strong trucks brand and frankly have underperformed in cars, Batey said. It's as simple as that. You have to go back a long way to see a car performance this strong.

The last time Chevy cars outsold trucks for three straight months was in May, June and July of 1991, when they made up 52 percent of sales for the year. Chevy is benefiting now from strong demand for its Cruze small car and Malibu mid-size sedan.

However, Batey sees the historical pattern of stronger truck sales in the second half of the year holding true for 2011, noting a steady increase in truck sales over the last two to three weeks.

Although the cars will remain robust and strong, I expect the trucks will come back somewhat in the second half of the year, he said. He forecast cars' full-year share of Chevy sales at 42 percent to 45 percent.

Long term, Batey sees Chevy sales at a third each for cars, crossovers, and the pickup and SUV segment. However, he said 2012 would get a boost from a full year of the Sonic subcompact, which goes on sale this fall, as well as the introduction of the new Malibu early in 2012 and the Spark minicar later in the year.

For the first six months, Chevy car sales will be up 23 percent, Batey said. Sales of pickup trucks and sport utility and crossover vehicles will show a 9 percent rise for an overall gain of 15 percent for the brand.

GM expects Chevy's retail sales to be up 26 percent for the first half. That includes gains of 42 percent for cars and 16 percent for trucks.

GM said Chevy was on track to sell 80,000 more cars and 40,000 more trucks through the first six months than a year earlier.

The Chevy brand has gained almost 1 percentage point in retail market share through April -- the latest period for which figures are available, GM said, citing Polk data. Cars are up 1.1 percentage points, and trucks up 0.7 points.

(Editing by Lisa Von Ahn)