A new financing plan for Seattle's proposed monorail will be announced today by a key adviser who says it won't require shortening the route from Ballard to West Seattle.

Kevin Phelps would not divulge the new numbers, in advance of his presentation tonight to the governing board of the Seattle Monorail Project (SMP).

Earlier, he predicted he could trim $3 billion to $4 billion from the SMP's former financing plan, which would have cost $11.4 billion in principal and interest over 50 years.

Phelps said he also will recommend more cautious forecasts for SMP's car-tab tax. The agency had estimated 6.1 percent annual growth based on a combination of rising city populations plus higher car values.

But that number was far higher than Sound Transit's planning figure of around 4.5 percent for its zone that includes Seattle, Shoreline and Lake Forest Park. SMP's numbers were challenged by regional economist Dick Conway, as well as monorail skeptics.

"My recommendation will be for a reduced [car-tax] forecast," Phelps said.

Among his possible ideas for cost reductions are leasing trains rather than buying them; paying for contingencies with a credit line only when needed, instead of selling bonds up front; and selling the bonds in several small batches, instead of a big $1 billion sale next year.

SMP board member Cleve Stockmeyer said he's hopeful the payoff period for the $2.1 billion line can be reduced to less than 40 years. He anticipates further debate about tax-growth rates in coming weeks, as part of the ongoing "civic discussion" in Seattle.

Phelps, a Tacoma resident, was formerly finance-committee chairman at Sound Transit, where he gained a reputation for asking hard questions.

Seattle Mayor Greg Nickels has issued a deadline of Sept. 15 for monorail leaders to propose a November ballot measure to either shorten the line or raise taxes, which have run one-third below original projections.

Before Phelps was hired, the agency and contractors already had cut several trains and three stations from the voter-approved plan of 2002.

They have recently considered shortening the line or "phasing" the construction. Phelps had considered delaying the northern section in Ballard two years, until 2013, but said yesterday that doing so wouldn't save a huge amount of money.

Deputy Mayor Tim Ceis said the Sept. 15 deadline remains firm, and Nickels will expect a "credible and defensible" car-tax estimate. He mentioned that the mayor is familiar with Sound Transit's lower numbers, and said that if the monorail's new figures are higher than those, SMP will have to justify them.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com.