The letter won't be sent unless a board majority signs off on the final wording.
Two board members, Karma M. Thompson, of the Salt Lake City Tesoro refinery, and Tammie G. Lucero, executive director of the Uinta County Economic Development office, opposed sending the letter, which is being refined by conference call before the EPA's July 1 comment deadline.
But a majority liked the idea of standards that would slash the key pollutants behind northern Utah's summer and winter pollution problems.
Using Cache County emissions as an example, Division of Air Quality staff showed how Tier 3 would cut nitrogen oxides and volatile organic compounds by more than half, even as the miles driven doubles in the next 25 years.
The two pollutants big contributors to Utah's summer ozone problem and winter smog would be cut by 80 percent, under EPA's proposal, and the fine soot that plagues northern valleys during winter inversions would be reduced 70 percent.
The cuts would be accomplished by better vehicle technology and cleaner fuel, gasoline with two-thirds less of the sulfur that makes catalytic converters burn less efficiently.
The air-quality improvements, EPA has said, will raise gasoline prices by a penny a gallon. The petroleum trade group American Petroleum Institute says it will be 6 to 9 cents more. It would also add around $134 to a new car's sticker price.
Lee Peacock, president of the Utah Petroleum Association, urged the panel to use "caution" before sending off the letter because of impact on refiners.
But vehicles account for roughly half of urban Utah's emissions pollution the state has no authority to regulate and northern Utah counties won't comply with the Clean Air Act without big emission cuts.
In the letter, the board called the standards "a potential crucial tool to assist in attaining and maintaining the National Ambient Air Quality Standards, protecting public health and maintaining and enhancing Utah's quality of life."
The environmental group HEAL Utah had asked the air-quality panel for the letter last month, and its policy director applauded the vote Wednesday.
"We're hopeful the board will send a strong message to the EPA that Utah desperately needs cleaner gas and cleaner cars if we're going to clean up our dirty air," said Matt Pacenza.