I can't prove it but I am sure that more than a few gas stations are rigging their pumps to not pump as much as the guage shows. this can be done easily as they mustkeep up with the ever changing gas prices. I did my own study with a 2002 Dodge 1200 pick up truck. From the day I bought it and for a perod of about 10 months, everytime I got gas I filled the tank up and recorded the milage driven. The milage ranged from 11 mpg to 14 mpg depending on the place of purchase.

gas pump efficiency is important since the gas prices depends on it. but it's too hard to detect which one is truly cheating over their costumers. there were some who uses gas saving device and I think it can be one necessary alternatives.

8-4-08 stopped at a BP.

Have a tank capacity of 19.5 gallons.

Receipt said I pumped 25 gallons for a charge of $100.00

My needle was on about 1/4 and when done it showed 3/4 so in fact I only pumped about a half tank.

That is approx. 10 gallons but even if the tank was dry, it is a physical impossibility for it to hold 25 gallons.

Comments?

Even if you get close to 10.0 gal, that should be good enough, and besides, it's not too difficult to correct for the error. In your example, 9.995 is only .05% off, so at $4/gal that would be $.002 and the dollar/cent amount shown on the pump is difficult to read to that level of accuracy.
You probably lose that much gas to evaporation and spillage. And again, a .05% error in your example weight of 18 lbs (3 gal of gas) is .14 oz, difficult to measure on most scales. Did I do the calc's right?
If somebody is going to risk getting busted for fiddling the pump, I would expect that s/he's going to fiddle it by a lot more than that, and any technique would catch it. They would be counting on people not even looking for it.

Also, isn't it also possible to change the amount of gas pumped? For instance, if the pump says 10 gal (or whatever) was pumped, isn't it possible that it actually delivered 9.5 gal? Or is the flow metering device sealed and can't be tampered with? Your weighing technique would be the way to catch that.

I tried taking a calibrated kitchen measuring cup of 1 pt, and using that cup "N" times, I put 5 gal of water in a gas can and marked the water line. Then I dumped it out, let it completely dry out, and filled it with gas to that same level and saw how much gas was pumped. In my case, it took less than 5 gal, so I was probably not accurate filling each pint with exactly a pint of water. Getting high accuracy is not easy.

Yeah, that's easier said than done. I can barely get the pump to stop at an even dollar amount let alone 10.000 gallons. Have you ever stopped it at say.... 9.995 gallons and been able to give it the lightest squeeze to get it to 10.000?? It's next to impossible.

I think science is the only true method for those 98% who aren't lucky enough to get it to stop at 10 gallons even. Calibrate a digital scale and weigh an empty 3-gal gas can. Add around 3 gallons, noting the exact gallons that the pump says it dispensed. At 60 degrees Fahrenheit, a gallon of vehicular gasoline weighs about 6.15 pounds at that temperature. So you multiply 6.15 by the gallons of gasoline to get a number. Weigh the gas can and subtract the weight of the can itself. The remaining weight should match the earlier number.

Peace!