PROVO — Anybody driving past the multitude of construction sites in Provo would have a hard time believing that Utah County's largest city is shrinking.

But if you believe the U.S. Census Bureau, that's just what happened to Provo in 2006. The city issued 558 new residential building permits, but somehow, according to census reports, the city shrank by 1,151 people.

Sound strange? It does to Provo Mayor Lewis Billings. That's why Provo, along with Saratoga Springs and Eagle Mountain, may challenge the lagging numbers.

"What do they know?" Billings said of the census bureau's latest estimate of his city's population. "We've just really found that their estimates are off. We evaluate (their numbers) and when it makes sense to make a challenge, we do."

Estimates by the census bureau put Provo's population at 113,984, but according to Billings, the city's actual population is closer to 117,000.

In Eagle Mountain, census bureau estimates say the rapidly growing city, whose population increased by 626 percent since 2000, has 12,232 residents, but city officials say the actual number is about 16,000.

Saratoga Springs' estimates say the city had some 12,000 residents in 2006, but the census bureau estimates the city had a population of 7,283.

Population estimates are used by state and federal agencies to distribute tax money back to cities and counties and plan for the area's future development. If those numbers are off, the cities will receive less money, and road and transportation plans for the area can be affected.

"We, as planners, use that data to plan in the future," said Shawn Eliot, Mountainland Association of Governments transportation planner. "If (the census data) is off, we don't plan the right-size roads and the cities have a hard time planning the right-size sewers. If our base data isn't correct, then our future data is skewed."

With Eagle Mountain's current estimated population of about 12,000 people, Mayor Don Richardson says the city receives about $1 million in taxes. However, Richardson says the city's estimated population done by city staff for 2007 is almost at 20,000 people. That means that if the city successfully challenges its census numbers, Eagle Mountain could gain nearly $1 million more in taxes.

"I think any city that goes through this much growth ought to be challenging the census," Richardson said. "These aren't normal growth rates. These are pretty high."

The census numbers in Eagle Mountain are unusually skewed because of how quickly the city has grown since 2000. Census bureau estimates that Eagle Mountain has fewer people per household and a higher vacancy rate than is accurate, Richardson says.