Who wouldn’t like to pay less for gasoline? That’s why there will be a lot of grumbling Wednesday when California’s gasoline tax goes up by 12 cents per gallon. That’s also why opponents of the tax increase are collecting signatures for a ballot initiative to reverse it, and are leading a drive to recall one state senator who supported it.

What isn’t get much attention in the debate is why our gasoline prices are already so high, averaging 60 cents above the rest of the country these days. Taxes play a role, as do programs to fight pollution.

But since early 2015 a large unexplained premium has been going to gasoline sellers. This may be profiteering, or it may be driven by infrastructure constraints on gasoline production and imports, but it is definitely not due to taxes.

Yet, politicians of all stripes have shown little interest in uncovering the cause of this surcharge, which has amounted to $12 billion — or $1,200 for a typical family of four — over the last 2 1/2 years. Those extra payments by California consumers continue today at a rate of about $3 billion per year, nearly twice the cost of Wednesday’s gas tax increase.

And unlike the extra tax revenue, this money is not going to fix any roads or bridges.

California has had higher-than-average gasoline prices since at least 1996, when we moved to a cleaner burning gasoline that costs an extra dime to produce. Research shows that the health benefits of our unique blend clearly outweigh that extra cost.

From 1996 until a Southern California refinery explosion in February 2015, the extra cost of gasoline in the state tracked closely with California’s higher taxes and production costs. Occasional refinery outages spiked prices, but they returned to the expected differential within a month or two, because that’s how long it takes to bring in our special blend of gasoline from refineries outside the state.

The 2015 market disruption, however, has been a different story. It’s been 32 months now, and Californians continue to pay at least 20 cents per gallon more than is explained by our higher taxes, greenhouse gas fees (cap and trade and the low-carbon fuel standard) and production costs.

Throughout that time, I was a member of the California Energy Commission’s Petroleum Market Advisory Committee, a panel of independent energy market experts, and I chaired the committee starting in August 2015.

The PMAC issued its final report last month. It highlighted the unexplained price premium that has hit consumers since 2015, but concluded that the committee had been given neither the authority, nor the resources, to fully investigate and determine the cause. The committee had no budget allocation, was assigned less than one full-time staff person, and had no authority to compel cooperation from industry.

It’s no secret that part of California’s higher gas prices go to reducing air pollution, fighting climate change, and maintaining roads to support a thriving economy. But it is a mystery why we are paying an additional $3 billion per year to sellers.

Before we roll back the new gas tax, before we cut the revenues for these public programs, shouldn’t we figure out where the rest of the money is going?

Severin Borenstein is a professor at UC Berkeley’s Haas School of Business and a researcher at the Energy Institute at Haas.