The old gas pumps, which are not really all that old, were strictly mechanical. They used gear ratios to have the dollars spent match the gallons pumped. But, as I think about this, what did they have - some wild machine shop? When prices started to really move, did the have a gear, and every combination of gears, for every occasion? How did they change this so quickly, too? What a pain it must have been! How did this really work? - Jinx 
now, before I get started, this is a memory from when I was 5 or 6 years old, so it may not be worth a bucket of warm spit, accuracy-wise. My stepfather used to own an old roadhouse/local hangout in South Dakota. He had one pump (mostly used when you needed a few gallons to get to the next nearest station, or for a lawnmower-it was a small gas tank, and the price was usually a bit more than a real gas station). Whenever he would change the price, he would go out, and open up the front of the pump (there was a panel, with a key lock). At the bottom of the display was the price per gallon display, which was the number wheels like the rest of the displays. I beleive all he had to do was manually change this to whatever he wanted to charge per gallon, and the rest of the machine was geared to charge accordingly.
Now, there may have been some sort of mechanism that had to be activated before changing it, I can’t remember.
Thanks, Lambo, but internally, wouldn’t there have to be different gear ratios needed so the gallons pumped would change slower and slower as the price per gallon rose? Likewise, wouldn’t the wheels of those digits showing dollars spent require gear changes as well to get the right gear ratio?
There must be some way the pump manufacturers simplified all this…
- Jinx
The following is hearsay, so don’t rely on it, but is offered in the hope of being of assistance.
As I understand it, the basic metering device on the (mechanical) gas pump is the one which kept track of the number of gallons (and fractions thereof) pumped on this particular cycle of operation.
There was a cam in the machinery which could be reset to turn the other display, the one showing cost of that pumping event, at any given ratio to the gallons-pumped display. Adjusting the cam made the third display, the price-per-gallon one, display a consistent figure, and would cause the other two to rotate at that fixed ratio. So if the bottom dial read that gas was 29 cents per gallon, the cam was adjusted so that each gallon displayed on the middle display would cause the top display to rotate by 29 cents worth – and 10 gallons would display at $2.90.
Back when I worked in a gas station, to change the price, we would open the front of the pump with a key. Under the price numbers were a set of ramps with notches, one for each number. There was a small lever that rested in whichever notch for each number. So if gas was 34.9 the lever under the tens was in the 3 notch and the lever under the ones was in the 4 notch. If we had a gas war and the price went down to 29.9 I would open up the pump move the lever under the tens place to 2 and the lever under the ones place to 9. Doing this would also move the numbers that showed through the front of the pump that the customer saw.
Yes, young ones it’s true, when I started in the gas station Premium was 34.9 /gallon and went on sale for 29.9. 
Used to fill up my small car for about $3, my parents big car cost about $10.
Since everyone has covered the change procedure, the lowest price I can recall is 24.9 at a chain of self-serve stations in the Carolinas, circa late 1960-early70. They had a bill reader into which you fed a $1 note, getting roughly 4 gallons.
Hah! I’ve seen gasoline at 17.9 cents per gallon, and beer at 15 cents per can. I remember 5 cent coke, 5 cent candy bars, and 5 cent pay phones. I remember cigarettes at 22 cents per pack. Heck, I remember penny post cards. Of course, if you made $200.00 per month, gross, you had a better than average job.
Oh, yeah? I used to go to the gas station (28.9/gal, IIRC) to get cigs from the vending machine for my dad. You put in a quarter, pulled a lever under the desired brand, and a pack would fall into a tray under the little window. Complete with three pennies change inside the cello wrapper. A box of matches would cost you one of those pennies, which most folks thought was kinda steep.
Oh yeah. The pricing mechanism was entirely mechanical, just as explained above.
Peace,
mangeorge
It should be noted that most or all of those mechanical pumps could not be set for more than 49.9 cents/gallon. This caused a problem when the first oil shock hit and prices went above that. As I recall, as an interim measure (until they could get new price wheels) they somehow changed the pumps to have the price be for the half gallon or liter. Don’t know how that managed to do that.
If this thread goes on much longer I’m going to have to point out that those are not pumps, they’re dispensers (or consoles). The pumps (down inna tanks) don’t care how much the gas costs.
Easy. if the price was, say 60 cents a gallon, they set the dispenser (peace) to 30 cents a gallon and you paid double whatever the dispenser said the total price was. If you bought 5 gallons, the dispenser said $1.50 and you paid the clerk $3.00. Remember that payment was manual too in those days, where even at self-serve somebody came out to look at the dispenser , see what you owed & take your card or cash.
A few places ended up going to triple-the-display pricing when gas got well above $1.00 and dispensers were still limited to prices below 50 cents. That only lasted a few months as dispensers with $9.99 pricing capability became available.
Back to the OP.
People have described how the station attendant operated the mechanism to change the dispenser’s price multiplier. But nobody has described how the price multiplier mechanism actually worked.
Polycarp mentioned a cam, but that’s a just term, not an explanation. (Not meaning to dis Polycarp, just pointing out that we still don’t have an answer yet)
Sadly, I have nothing to add on that point. I can readily imagine a simple device that’d do the trick, but it’d be real jumpy, where the total price drums would be rotating jerkily not smoothly as the gallons were dispensed. So I bet I’m on the wrong track. Besides, my idea would be mere conjecture on how it was done, not actual fact.
I asked the folks over at oldgas.com, a site for gas station antiques collectors.
Here is the forum page with the answers, one of which even has a picture of the mechanism!
I owned a 60’s era gas pump, and I can guarantee that the motorized pumping mechanism was in the above ground pump.
I see DD beat me to a picture. I searched the US Patent database and came up with this one:
Click Images for the drawings. An ActiveX file is req’d.
I dunno about that, Fear. You’re talking about here, on Earth, right? I worked on and around them things a lot in the sixties, old and new, and never saw one with the actual pumping mechanism in the same console as the pricinfg unit. Are you sure you’re not confusing the metering unit for a pump? It would take a pretty husky pump to bring the gas all the way up from the underground tank. They use submersible pumps due to safety considerations too.
I have seen really old, hand pumped, units. I’ve also seen metered dispensers connected to above-ground tanks which were gravity fed. Those were mostly in commercial situations though, such as on large farms, which is why they were usually like 20ft above the ground.
I could surely be wrong, though. Somebody enlighten me, please. 