General Motors Co passed an environmental milestone of sorts in March, when it became the first U.S. automaker to sell, in a single month, more than 100,000 cars that can achieve 30 miles per gallon in highway driving, the automaker said on Monday.

About 40 percent of GM's March sales were of these fuel-efficient cars, said Mark Reuss, GM president for North America.

Other major carmakers including Ford Motor Co and Hyundai Motor Co <005380.KS> have also dramatically increased their sales of fuel-efficient cars in the wake of soaring gasoline prices.

Analysts say that GM has come a long way since President Barack Obama was quoted as saying about American automakers in November 2008 - in a 2010 book by Steven Rattner, former head of Obama's auto task force - Why can't they make a Corolla?

GM has made a complete turnaround where fuel-efficiency is now a priority compared to just a few years ago, said Jesse Toprak, an analyst at TrueCar.com. As consumers have started to request more fuel-efficient cars, manufacturers have done a really good job to meet their demands.

GM's Reuss said that in recent years, the automaker has invested in four-cylinder and turbocharged engines, more fuel-efficient transmissions and hybrid and plug-in electric cars.

Three years ago, about 16 percent of the vehicles GM sold achieved at least 30 mpg on the highway, Reuss said in a statement on Monday. Today, that number is 40 percent.

Auto consultant and analyst TrueCar.com compiled its own estimates of automakers' U.S. sales and the percentage derived from models that attain at least 30 mpg in either highway or city driving.

The estimates show that U.S. automakers have improved greatly on fuel economy since 2008, and that manufacturers are better-prepared for the current spike in gasoline prices than they were in July 2008 when gasoline reached a record high average U.S. price of $4.11 a gallon.

The U.S. government said on Monday that the average price for regular gasoline rose 2 cents in the past week to $3.94 per gallon.

Last month, according to TrueCar.com, 43 percent of GM's U.S. sales derived from vehicles that achieve at least 30 mpg. That compares with only 16 percent of GM's monthly sales four years ago, TrueCar.com said.

Ford's 30-mpg models accounted for 36 percent of its March sales, up from 11 percent in 2008, and 16 percent of Chrysler's U.S. sales in March were expected to come from fuel-efficient vehicles, compared with zero four years ago.

The leader on that list was Korea's Hyundai at 83 percent in March, up from 60 percent in 2008.

Major automakers will report March U.S. sales on Tuesday. Thirty-eight analysts surveyed by Reuters forecast an annualized sales rate of 14.75 million vehicles, up from 13.1 million a year earlier.

VOLT SALES REBOUND

March sales of the Chevy Volt, the plug-in electric hybrid that is a halo car for GM, will be the highest for any month since its December 2010 launch, a GM spokesman said on Monday. Halo car is industry jargon for a unique automobile intended to draw attention to an overall brand.

The heavy sales month for Volt comes amid a work stoppage after the Volt's sales were disappointing through February. GM did not offer specific sales figures for the Volt, but said they would top December 2011 when sales were 1,529 vehicles.

GM has halted Volt and Opel Ampera production at its plant in Detroit and the neighboring city of Hamtramck, Michigan, for five weeks, until April 23. The Opel Ampera, which is basically the same car as the Volt, is sold in Europe.

The company continues to work to adjust Volt production with market demand, the spokesman said.

Last November, the National Highway Traffic Safety Administration opened an investigation into the safety of the Volt's battery pack after its own tests uncovered fire risks. By January, NHTSA closed its probe without finding any defects and said it was satisfied with GM's fix to better protect its lithium-ion battery pack.

The federal government estimates that Volt can achieve a gasoline efficiency equivalent of 93 mpg.

GM battled the fallout of the NHTSA probe by offering leases of the Volt at $349 per month, matching its introductory offer in early 2011. About 40 percent of Volt sales are linked to leases as opposed to about 25 percent before the NHTSA probe.

The Volt's sales of 8,000 in 2011 are paltry compared with sales of GM's gasoline-efficient models.

GM's answer to Obama's Corolla question is the Chevrolet Cruze, which recorded sales of 231,732 vehicles in 2011, second in the small sedan category only to Toyota Motor Corp's <7203.T> vaunted Corolla, with 240,259 in sales, according to Autodata Corp.

On Sunday, NHTSA opened an investigation into reports of two cases in which 2011 Cruze models had engine fires.

For February, the last full month of official new auto sales posted, among small cars only, Toyota had the best average fuel economy in the U.S. market at 35.3 mpg, followed by Honda at 32.3 mpg, Ford at 32.2 mpg, Hyundai at 31.3 mpg, Nissan Motor Co <7201.T> at 30.8 mpg and GM at 30.3 mpg, TrueCar found.

(Reporting By Bernie Woodall; editing by Matthew Lewis)

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