When BART officials rolled out their $3.5 billion bond measure planned for the November ballot, they said repayment would cost the average homeowner about $35 to $55 annually for 30 years.

In fact, the average annual property tax for a home would be roughly twice that. And it would last for 48 years, according to BART’s own projections.

But don’t expect any tax amount to appear on the ballot. In a cynical political calculation, BART directors and staff decided to only mention the size of the bond issue and the sorts of capital projects for which the money would be spent.

Unlike state bonds, which are usually repaid from existing revenues, local bonds typically require new taxes to cover the debt. In this case, the language BART plans for the ballot makes no mention of the property tax increase.

Matthew Burrows, BART’s general counsel, gave the board two reasons for the omission. First, he said, with the ballot language limited to 75 words, there wasn’t room to mention the tax. That’s bogus. Second, the planned ballot wording had tested best in a voter opinion poll. In other words, the political ends justify the means.

In fact, BART officials never considered ballot language that mentioned the tax, said Kerry Hamill, assistant general manager for external affairs. Nor did they include it in their polling.

The ballot measure in the three BART counties requires two-thirds approval. BART officials know that voters informed about the tax amount would be less likely to support the bond. Since no law requires BART officials to put that information on the ballot, they won’t.

For tax information, voters will have to dig deep in a voluminous election guide they will receive in the mail. BART officials know many won’t bother.

Those voters will rely on the incomplete ballot language and perhaps news accounts, which understated the tax because of bad information from BART.

Specifically, Hamill and Rose Poblete, the district’s controller-treasurer, misinformed the board and the public about the amount of the tax. At a May 26 board meeting, Director Tom Blalock asked Hamill how much the tax would be.

“The average cost per household per year would be between 35 and 55 dollars per year over the life of the bond,” Hamill said.

“I assume they’re 30-year bonds?” Blalock then asked. To which, Hamill then replied, “uh huh” and nodded.

That information was interpreted to mean that the tax would last for 30 years, and that’s what reporters disseminated in June, when BART voted to place the bond measure on the November ballot. But it’s wrong.

Agency documents show the tax lasting for 48 years, not 30, beginning in 2018 and ending in 2065. That’s because BART plans to spread the bond issues over nearly two decades as the money is needed. Each issue would then be repaid in the following 30 years.

As for the amount of the tax, Hamill relied on a document Poblete provided showing the estimated average annual levy for an average home in each of the three Bay Area counties.

The complex calculation relies on annual tax rates derived by estimating the bond repayment amount each year and forecasting for the next half century the total assessed value of property in the three counties.

Poblete then applied those annual tax rates through 2065 to a home in each county with a current average assessed value. But she did not increase the value of that home over time.

Consequently, she reported that the “estimated average annual tax” would be $37 in Contra Costa, $39 in Alameda County and $52 in San Francisco. That roughly approximates the average tax in current dollars. But that’s not how it was presented.

In future dollars, assuming, for example, a 3 percent annual inflation on that home, the average annual tax through 2065 would be $79 in Contra Costa, $83 in Alameda County and $111 in San Francisco.

It’s been more than two weeks since I pointed out the error to Hamill and Poblete. They have yet to issue a public correction.

Daniel Borenstein is a staff columnist and editorial writer. Reach him at 925-943-8248 or dborenstein@bayareanewsgroup.com. Follow him at Twitter.com/BorensteinDan.