The California Air Resources Board acknowledged something Tuesday that critics have been saying for months: the state vastly over-estimated the amount of diesel pollution emitted by big off-road construction vehicles.

The error, contained in an ARB computer model and compounded by a recession that idled far more trucks than expected, means that the construction industry would come close to meeting state-mandated targets for reducing pollution through 2025 even if regulations designed to force firms to retire or retrofit their dirtiest trucks are repealed.

The rule affects more than 100,000 construction vehicles, including tractors, scrapers, graders, bulldozers and other trucks used on road and building projects around the state.

The revision was prompted in part by an independent study showing that actual diesel fuel sales in California to construction companies were far lower than the state’s computer model predicted.

After reviewing the model, the ARB’s staff conceded that there were fewer construction vehicles in operation than projected, those trucks were newer and thus cleaner than anticipated, they were being used less often than expected, and when they were used, they were run at a lower power than the model assumed.

Combined, all of this means that construction vehicles are polluting far less than the state believed when the ARB adopted regulations in 2007 to reduce diesel pollution from the industry.

The agency is holding a series of workshops around the state to solicit input from stakeholders and others on the revised emissions projections, known as an inventory.

Once those workshops are complete, the ARB staff will propose a revised regulation reflecting the lower numbers. The agency is expected to act on that regulation by the end of the year.

Industry officials said Tuesday that the new figures are so low that the state should simply repeal the regulation and let the industry’s fleets grow cleaner on their own as older trucks are retired and replaced by newer vehicles with engines that burn cleaner.

“The fact that this agency has been willing to find and fix the significant flaws in its original estimates is a victory for sound science over rash regulation,” Mike Kennedy, general counsel for the Associated General Contractors of America, said in a statement released by the association. “As the agency’s own data now makes clear, it is time for the board to repeal its costly and unneeded rule.”