NEW YORK (TheStreet) -- Ford (F) - Get Report CEO Mark Fields said the 2015 is still shaping up as a "breakthrough year" for Ford -- even as the company posted a first-quarter profit of 23 cents a share, missing analysts' expectations of 26 cents. But those analysts made erroneous assumptions about the automaker's tax rate. First-quarter profit slipped 6.6% from a year ago, as Ford continued to ramp up production of its most important profit-making model, the new F Series aluminum-body pickup truck. 

Bob Shanks, executive vice president and chief financial officer, said in an interview that Ford had guided analysts with respect to the tax rate during the New York auto show earlier this month. Nevertheless, analysts had been expecting a 29% tax rate, according to Bloomberg; Ford posted a 34% tax rate.

The other big Detroit-based automaker, General Motors (GM) - Get Report, also missed analysts' profit expectations last week on weak results in Russia and South America. 

"The action will be skewed toward the end of the year," Shanks said, referring to Ford's profit. He said the automaker is introducing new models, such as the Ford Explorer SUV and a three-row Ford Edge in China. By the second half of the year, Ford should be building F-Series pickups at full speed at two U.S. assembly plants in Dearborn, Mich., and Kansas City, Mo. 

In a statement, Fields forecast that Ford's full-year pretax profit will grow by as much as 50%.

"The first quarter was a good start to a year in which our results will grow progressively stronger as the new products we have been launching start to pay off," said Fields in the statement. "We are re-confirming that 2015 will be a breakthrough year for Ford."

The company has said profit margins in North America are improving due to consumer enthusiasm for the F-Series, even as business conditions have been poor in South America and Russia. Operating margins should be 8.5% to 9%, up from previous guidance of 8% to 9%. 

By switching from a steel body to aluminum for the F-Series, Ford took a big gamble with its most important vehicle. But analysts generally have been satisfied with the rollout of the new model, though the changeover has taken a short-term toll on Ford's financial results. A 700-pound reduction in weight has resulted in 29% better fuel economy, with no apparent loss in durability. For 33 years, the F-Series has been the industry's single best-selling vehicle model in the U.S. 

Relatively low prices for gasoline are a big reason why consumers are choosing pickups and larger-sized vehicles. Interest has been waning in small vehicles and alternative-fuel models such as hybrids. 

Ford last week announced it was cutting 700 jobs and one work shift at its assembly plant in Wayne, Mich., meaning that production will be lowered for its Focus small car and C-Max gas-electric hybrid model. 

The entire industry has been affected by weak energy prices. In the first quarter, smaller vehicles represented 18.5% of the U.S. market, compared with 20% a year ago. That's the lowest proportion of small cars in six years. Sales of electric and gas-electric hybrids were down 16% for the quarter.

This article is commentary by an independent contributor. At the time of publication, the author held no position in the stocks mentioned.