Someone told me yesterday that they thought that gasoline pumps were rigged to pump less than what is indicated on the display. That piqued my interest for two reasons, one because I don't like getting cheated, and the second, because in a past life, I was a water meter guy.

Between 1985 and 1993, I worked for Schlumberger Industries, Water and Gas Group, Technical Services Division. I traveled the U.S. installing, testing and calibrating water meters ranging in size from 5/8" - 1.5" (residential), 2" to 12" commercial and industrial, all the way to 60" behemoths that you would find feeding mejor cities.

In that line of work, when calibrating a meter, the acceptable range depended on the type of meter. Smaller meters, 5/8" through 2" are almost always positive displacement types which used either oscillating or nutating discs. They were very good at capturing low flows and were considered accurate if they registered 95%-101.5% at the lowest flow rating and 98.5 - 101.5 at the highest flow rating. My assumption is that the meters in gasoline pumps are also of the positive displacement variety but I am not sure exactly what the technology is. If anyone knows, please provide details.

Anyway, I googled gasoline pump accuracy and found many articles on the subject. I was able to find that at least in a few states, the acceptable accuracy is 6 cubic inches per five gallons. That translates into +/- one half of one percent. So with $2/gallon gasoline, the acceptable accuracy is +/- one cent.

According to the articles that I read, gas pumps are checked by the state as frequently as once per year, and as infrequently as once every four years. It does look like improvements have been made over the past several years however, with failure rates dropping significantly. It was also interesting to note that there was no trend toward cheating the customer. Failures were evenly distributed between overcalibration and undercalibration.

Go figure!

 

Hi Dan,

Interesting post, thanks.

For those who have not undergone the proper indoctrination, there are 231 cubic inches in one gallon. I was not aware such a unit was used, but google knew ..

So the accuracy is 1/192.5 -- about half a percent as Dan wrote. Or up to two US pennies for a Prius fill of 10 gallons.

btw, congrats on the *brilliant* FE.

 

I know here in Ohio the Mayor has a sticker on each of the gas pumps, saying they have been calibrated and are accurate per the "Dept of Weights and Scales", or something like that. I have two or three stations I like to frequent and notice that they are checked yearly, or at least have the current year sticker. I will look more closely to see if it says what the actual acceptable accuracy percentages are.

 

the sticker will probably not say. But the responsible department's website probably does, or at least has the info on where to find out.

 

In Oklahoma, the Corporation Commission's 23 inspectors inspect and certify pumps-- some 50,000 pumps in all-- and twice a year!

 

Around here the stickers are signed by each cities "Sealer of Weights and Measures". Quite often I have noticed that the last name is the same as a prominent politician in that city or town.....I am sure that is a coincidence. In other words, I trust the pump more than the person testing them. Think it is done once a year here.

 

In both Texas and Florida, the Agriculture Dept.(!) has a division that tests all gas pumps for accuracy (at least annually, IIRC). If any pump puts out less than what is shown on the display, that gas station is in big trouble!