The federal government spent $13.5 million on cars and SUVs it has no idea whether it needs, according to a new audit from the Government Accountability Office.

The report found that government workers also billed taxpayers $2.5 million for vehicle upgrades such as heated seats and video entertainment systems.

The watchdog group found that the government spent $1.6 billion to purchase over 64,000 new vehicles between 2011 and 2015, at an average cost of $25,600. The majority of purchases were made by five agencies, the Departments of Defense, Homeland Security, Agriculture, Justice, and Interior.

The GAO found that some agencies are failing to keep track of whether vehicles are used. For instance, the Customs and Border Protection agency could not determine if 81 percent of its vehicles were used in 2015. Over 1,800 CBP vehicles were driven less than 12,000 miles that year, the minimum mileage required by Homeland Security to justify purchasing a vehicle.

By contrast, the Navy was able to account for all of its 3,652 vehicles reviewed in the GAO report.

The GAO concluded that between just two agencies, the CBP and Natural Resources Conservation Service within the Agriculture Department, the government spent $13.5 million on depreciation and maintenance costs for vehicles that neither agency could say were used.

"Federal agencies spent about $3.4 billion in fiscal year 2015 to keep and operate almost 450,000 federally owned vehicles," the GAO said. "Each federal agency is responsible for determining utilization criteria and assessing vehicle utilization."

"While these costs may not equal the cost savings agencies derive from eliminating underutilized vehicles, without corrective action, agencies are incurring expenses to retain vehicles without determining if they are utilized," the GAO said.

Federal agencies also billed taxpayers for questionable upgrades for cars and trucks.

The government spent over $2.5 million on upgrades that the GAO could not determine were necessary for the agency's mission, as required by federal guidelines.

Upgrades included heated or leather seats, remote keyless start, powered seats, and video entertainment systems.

"In analyzing these options, we were not able to determine if six of these types of options were related to safety, efficiency, economy, suitability, or administrative functions," the GAO said.

Six vehicles had video and sound systems installed worth over $3,000 each, totaling $18,524. Heated or leather seats cost an average $1,300 per vehicle, totaling $49,476.

Some upgrades were justified, the GAO said, such as a $167,427 souped-up truck the Army uses for recruitment. The Ford pickup is equipped with leather, heated, and cooled captain chairs, DVD and MP3 players, a Sirius radio, chrome door handles, custom head lamps, and polished aluminum 22.5-inch wheels.

Army officials told the GAO the truck has been a hit at more than 100 events and is one of the most requested assets in its fleet. The truck was purchased to replace six Hummers.