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(WBIR) There are about 90,000 gas and fuel pumps in the state of Tennessee, each one dispensing 134,000 gallons per month.

However, how do you know when you pump a gallon you actually receive that amount? The state Department of Agriculture employs 20 inspectors to do that job, making sure each pump is annually checked.

"(We) check on safety and calibration," Terry Chaney, the East Tennessee weights and measures supervisor said.

His job is to test stations' fuel octane, check prices, make sure the pumps are safe, and make sure they accurately dispense a gallon of gas.

"We check on a routine inspection, we check 15-percent of the pumps – randomly inspect them. And if everything's all right, then we approve the station," he added.

Earlier this week, Chaney tested gas stations in Jefferson County. At each station, Chaney performed a surprise inspection.

"I'm going to check three pump. If you'll turn them on for me when I get there," he told an employee at the Fastop in Dandridge.

At each station, Chaney tested multiple, pre-selected pumps, and filled a metal bucket up to five gallons. If the pump dispensed five gallons, give or take six cubic inches, it passed.

He tested one pump: "It's one cubic inch above zero. Our tolerance is six cubic inches, so it's well within tolerance," he said.

That margin of error is not very big -- just a few drops could mean the difference between pass and fail -- leading to expensive repairs for the owners of the gas station.

"As you can imagine, most of these gas pumps are in use continuously every day and they wear. Most of the problems we find are wear and tear," said weights and measures administrator Robert Williams.

About five percent of all pumps fail in some facet, whether it's improper measurements, safety issues, or dispensing different octane grades to customers.

"Most of the time, what we do if there's a problem with the pump, what we'll do is we'll tag the pump and make the consumer and the business owner aware there's a problem with it," said Williams. "We allow a reasonable time to have it repaired. If it's severe enough, we will remove it from service right there."

If the pump passed, a blue and white sticker with the state seal was added to it. This means new certification and reassurance.

"It's just an important program that we have with the state," Chaney said. "Making sure the consumer gets the correct amount."

Knox County's results

10News requested the 2013 statistics of gas stations that failed because of calibration issues. The statistics showed 149 pumps at 32 stations in Knox County had failed pumps.

Results show the customer actually benefits from the inaccuracy more than getting ripped off. Twenty-five pumps gave less per gallon of gasoline, beyond the six cubic inches margin of error. Of the 149 pumps with errors, 124 pumps gave more gas per gallon, benefiting customers.

Below is a map of stations with errors in Knox County.

[View full screen version of map]

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