In Ohio (and, IIRC, Penna., Vt. and N.Y.), gas pumps have little stickers - sometimes with the name and picture of the county auditor or other responsible functionary - on them, declaring that the pumps have been “inspected and sealed” by that worthy or his/her minions. Inspected, maybe - but sealed? What is there, really, to keep gas station owners from fiddling with the pumps to wring out a few extra pennies for every gallon sold, which could amount to big bucks over time? Do the stickers really serve any necessary function?
The seals make it legal for the pumps to measure the gas. Go into a store and you’ll see the same seals on their scales (BTW the county charges the stores for the seals. I don’t know about where you are, but by me it costs $25 per scale). Technically, those are the only scales we are allowed to use to figure out how much to charge someone for something sold by weight. You’re right, there’s nothing preventing the owner from adjusting the pumps afterwards, but if you ever feel that a pump is ripping you off, just call the city and they’ll send someone over to check it out.
If you go over to fox6news.com and click on the left side on investigations, there’s an article called “Hosed at the pump.” Milwaukee’s Fox station just did a special on this and you can read it here, at least until they pull it from the site.