Superior Industries International Inc. has issued a letter to shareholders urging them to vote for the company’s slate of directors in an ongoing proxy fight. The board of the Van Nuys company, which manufactures aluminum wheels for cars and pickups, has dealt with competitive elections for the last three years from Gamco Investors Inc., its largest shareholder with a 12.6 percent stake in the company. The New York investment firm, controlled by Wall Street billionaire Mario Gabelli, has argued that the current board has not developed the full value of the company, and the addition of financial expertise will help realize that goal. Superior, in its letter, defends its strategic moves, including moving manufacturing to Mexico, and the quality of its current board. “Despite having its director candidates rejected by shareholders for the last two years, including last year by a significant margin, Gamco has again proposed an alternative slate of director candidates,” the letter released Monday states. “We question Gamco’s choice of nominees to replace three of your board’s most highly qualified, experienced and valued directors.” The three sitting board members Gamco seeks to replace are Paul Humphries, president of High Reliability Solutions, a subsidiary of publicly traded Flextronics International Ltd. in Singapore; Timothy McQuay, managing director of investment banking at Noble Financial Capital Markets in Boca Raton, Fla.; and Francisco Uranga, chief business operations officer for Latin America at Foxconn Electronics Inc. in Taipei, Taiwan Gamco’s alternates include Walter Schenker, a former stock analyst who Gamco has nominated unsuccessfully for the last two years; Philip Blazek, president of Dallas investment firm Special Diversified Opportunities Inc., who was nominated by Gamco last year; and Glenn Angiolillo, a corporate lawyer. Votes are due by Superior’s annual shareholder meeting, scheduled for May 5 in Southfield, Mich. Shares closed up 11 cents, or less than a percent, to $19.46 on the New York Stock Exchange.