A trio of car rental companies at San Luis Obispo County Regional Airport has underpaid close to $29,000 in rent to the county, according to an audit by the Auditor-Controller’s Office.

Avis was short by $15,235; Enterprise by $7,565; and Hertz by $5,877, according to the report, which was presented to the Board of Supervisors on Tuesday.

The audit was part of a periodic examination of the six concessionaires at the county airport.

The report said the shortages were due to “eliminating surcharges, discounts, vehicle licensing fees, government administrative rate supplements, underage fees, and drop fees from gross revenue.”

Auditors found nothing amiss with payments involving the other three concessionaries, although they discovered minor problems with such matters as putting the proper date and time on some transactions.

The other concessionaires profiled were Spirit of San Luis restaurant, Budget Rent-a-Car and Alexis Enterprises, which ran a snack bar and gift shop at the airport.

The concessionaires combined generated $566,277 in rental income for the fiscal year 2010-11, according to the report to the Board of Supervisors from Auditor-Controller Jim Erb.

Erb said it will be up to the airport manager and the General Services director to work out payment schedules for the businesses.