By now, you've seen the 12-cent per gallon spike at the pump because of the new gas tax law, but could California tax drivers per mile instead?

The California State Transportation Agency released a final report earlier this month that at least entertains the pay-per-mile tax, which would replace the current gas tax law.

Caltrans spokesman Mark Dinger said cars are getting more fuel efficient all the time. That means people filling up less often.

"The gas tax is just not sustainable in the long-term," said Dinger.

The agency is studying technology, whether that's attaching a device inside someone's car to using a smartphone application, to track someone's mileage. According to the report, fuel efficiency will impact the state's ability to collect revenues to maintain road infrastructure.

Nonetheless, the report states that a pay-per-mile tax plan wouldn't be implemented until at least 2025.