It's painful enough to fill up the gas tank nowadays, but imagine how you'd feel if you weren't getting all the gas you actually paid for.

That's exactly the sort of situation state investigators want to help customers avoid by inspecting gas pumps and testing them for accuracy.

When you're paying nearly $4 for a gallon of gas, every nickel and dime matters.

On Tuesday KOMO News tagged along with state inspectors as they went from gas pump to gas pump to make sure all the money you pay end up in your tank.

"We always do our inspections unannounced," said Kirk Robinson with the state Department of Weights and Measures.

Inspectors use a precision 5-gallon tank to make sure consumers get what they pay for.

"This here measure it. If it's right at zero, that means it's 5 gallons. This pump here was giving away a little bit of gas, about one cubic inch," said Robinson.

One cubic inch equals a little more than half an ounce. Pumps must be accurate within 6 cubic inches.

In the first three months of the year, inspectors checked more than 1,800 pumps. Ninety-four percent of them passed; 6 percent didn't. The 115 pumps that failed were taken out of service.

"Our inspector will come back in and verify that that pump is accurate, kind of do a recheck on that," said Robinson.

If you have any doubts about what the pump reads the next time you fill up the tank, look for a sticker left behind by the inspector. It'll tell you when the pump was last inspected, and the number to call to complain.

Robinson says as gas prices continue to climb, so do the number of complaints the department receives.

"We're up almost 60 percent in complaints over this time last year," he said.

Inspectors plan to test some 3,500 more pumps by Memorial Day.

If you have a complaint about a gas pump, you can contact the Department of Weights and Measures by calling (360) 902-1857 or by e-mailing wtsmeasures@agr.wa.gov.