Drivers who fill up their gas tanks believing they’re pumping 10 percent ethanol may be surprised to learn it could be much more.

The resulting damage can mean costly car repairs.

Experts blame a process called “splash blending,” which often doesn’t thoroughly mix gasoline and ethanol.

Splash blending is frowned upon in other countries such as Britain, but it’s widely practiced in the U.S. It’s a challenge for state regulators, who only check fuel makeup periodically.

Such checks should come more frequently, especially when the price of ethanol drops below that of gasoline.

The Kansas City Star reported Sunday that while it’s not known how much excess ethanol is finding its way into our fuel tanks, people who conduct fuel tests frequently find proportions that are badly out of whack.

For example, the service manager at a Lawrence, Kan., car dealership complained of low mileage. The cause: His tank had 20 percent ethanol.

Splash blending occurs when the ethanol and gasoline are pumped into a tanker truck or filling station storage tank separately. At wholesale terminals, fuels are more likely to be pre-blended.

If they’re not mixed thoroughly, they can stratify into layers. Similar problems can occur with biodiesel, where vegetable oil and animal fat are mixed with conventional diesel.

The overall contents of a storage tank or tanker may be 10 percent ethanol, but individual customers may end up with much more than that in their gas tanks.

Since ethanol doesn’t have the same energy content as gasoline, the result is lower fuel mileage. At high levels, excess ethanol can cause damage to catalytic converters and engine parts, including fuel pumps.

Gasoline prices have dropped recently. But when ethanol is cheaper than gasoline, the fuel industry has an incentive to put in too much of the cheaper fuel, using splash blending.

In an Iowa case, the attorney general sued one retailer for selling fuel with 18 percent ethanol. The retailer had been telling customers the fuel’s ethanol content was zero. Kansas tests the ethanol content of retail outlets every 18 months. In Missouri, the checks take place every 20 months. Officials say they haven’t found any problems.

But an Atlanta company that sells fuel-test devices lists Missouri as among the six states with the most reports of excess ethanol.

When the price of ethanol dips below that of gasoline, the frequency of fuel tests by state officials should increase. At the same time, state lawmakers should look at whether regulators are doing enough to protect consumers.

As Todd Sneller of the Nebraska Ethanol Board put it, “The consumer has a right to know that if E10 (10 percent ethanol) is what is on the pump, that is what is being sold.”