"You could be getting ripped off at your favorite gas station," cries the first line of this NewsWest 9 online story. The rest of the article continues with the same tone which is that the consumer is always getting short-changed.
But they also provide a spreadsheet of the findings that you can download and have a look at.
Correct me if I am wrong, but it looks to me (based upon this spreadsheet) that the consumer is just as likely to get more gas than they paid as less.
Any mechanical pump makes errors. But sometimes they are in your favor, too.