About 40 applicants are on the rebate waiting list, Ballard said.
Since the rebate money comes from the CNG surcharge, it takes some time before the fund replenishes and rebates can be paid again.
Ballard said high-profile efforts such as Gov. Mary Fallin's CNG fleet purchasing plan have helped raise the awareness of the benefits of CNG vehicles.
Honda, the only automaker to offer CNG vehicles from the factory, also offers a $3,000 CNG fuel card with the purchase of a new CNG Civic.