It is an industry maxim that consumers who consider buying a Toyota in the U.S. almost exclusively cross-shop Honda, and vice versa.

However, despite Toyota Motor Sales U.S.A. Inc.’s recent unintended-acceleration woes, American Honda Motor Co. Inc. has lost market share in the U.S. in 2010, signaling it is not benefitting as much from Toyota’s share slide as other competitors.

Toyota’s slice of the U.S. light-vehicle pie stood at 15.4% through April, down from 16.1% in like-2009.

Including both Acura and Honda brands, Honda’s U.S. share has slipped to 10.5% in the same period, down from 11.0% year-ago, Ward’s data indicates, in spite of an 11.5% increase in sales volume.

The Honda brand is to blame for the entire 0.5-point drop, with Acura’s U.S. market share holding steady at 1.1%.

While Honda-brand penetration rebounded in April from March, rising to 10.5% from 9.1%, the April figure remains below the 11.3% share of the U.S. market it commanded in April 2009.

Two vehicles in particular are behind the decline, the midsize Accord and compact Civic.

The Accord controlled 5.3% of the total car market through April, vs. 5.6% year-ago. The Civic’s penetration slipped to 4.4% from 5.1% in like-2009.

The two models are Honda’s highest-volume vehicles, with the Accord the No.2 best-selling car in the U.S. this year and the Civic close behind at No.4.

On the light-truck side, the Odyssey minivan and Element cross/utility vehicle each have lost 0.1 points of market share from year-ago.

The greatest beneficiaries of a stumbling Honda and Toyota are Ford Motor Co. and Nissan North America Inc.

Ford is the biggest gainer in the U.S. light-vehicle market through April, taking a 17.1% slice of the sector, up from 14.9% year-ago.

Nissan’s share has jumped to 8.3% from 7.4% in like-2009, with the entire increase attributable to the mass-market Nissan brand.

General Motors Co.’s Chevrolet brand has moved up to 13.4% from 12.0% in the same period, while Subaru of America Inc. has continued its sales and market-share streak into 2010. After posting the biggest sales gain in the U.S. last year, Subaru’s volume is up 41.1% through April; and its market penetration has risen to 2.3% from 1.9% year-ago.

The Volkswagen brand has seen its share rise, as well, to 2.3% from 1.9%.

While Hyundai Motor America has gained just 0.1 points of share this year, data from Web analytics firm Compete Inc. indicates the new Hyundai Sonata midsize sedan, along with Ford’s Fusion, may be hurting the Accord.

Compete data shows cross-shopping of the Sonata and Fusion by Accord shoppers has surged to or near record highs.

“Essentially, Ford Fusion, as well as the Hyundai Sonata, is of much more interest to Honda Accord shoppers than it has been in the past,” says Dennis Bulgarelli, director-Automotive for Compete. “Both of those now are Top 10, if not Top 5, models shopped by (Toyota) Camry and Accord people.”

American Honda spokesman Chuck Schifsky shrugs off 2010 market-share losses, saying the auto maker sets sales, rather than share, targets. He points out American Honda sales were up through April, gaining 9.7% according to Ward’s data.

“Market share isn’t always a good indicator of success,” he says. “In late 2008 and 2009, our market share went up, even though our sales went down – due to the fact other car companies’ sales fell at a quicker pace than ours. We didn't fall as far (last year), so our gains (this year) won't be as dramatic.”

Schifsky says Honda is happy with the Accord. Sales of domestically built models, the vast majority of Accords sold this year, are up 24.6%.

Bulgarelli notes interest by Accord buyers in the Sonata, in particular, may be short-lived, if history is a guide.

“Remember when the (Chevy) Malibu came out? That got a lot of attention from Accord and Camry people too, but that level of interest wasn’t necessarily maintained in the long run.

“We’re seeing it maintained for Fusion,” Bulgarelli continues. “The question is will Sonata be able to maintain that as well?”