# How fine is product delivery on a gas pump?

Whenever I put gas in my car I try to get a perfect round number to pay: as in \$10 or \$20. The pump works so fast that to get the last few cents worth I have to just barely click the pump lever to catch the last few cents worth.

Whenever I do this I wonder: Is the calibration on a gas pump fine enough to deliver the product this accurately?

Am I actually putting 2 or 3 cents worth of gas into my tank or am I simply playing with the pump gauge to get a number I want?

If gasoline is selling for \$3.00 per gallon, then one cent’s worth of gasoline is 12.6 cubic centimeters, about 3/4 of a cubic inch. That’s an easily measurable quantity by a positive-displacement pump-meter. Imagine a gerotor oil pump or a bent-axis multi-piston hydraulic pump; you know exactly how much fluid is being moved for one revolution of the shaft, and you can get digital shaft encoders with extremely fine resolution, 3600 pulses or more per revolution.

I don’t know what kind of measurement resolution is present on fuel dispensers, but based on the above I have no problem believing it’s possible to measure gasoline in sub-penny quantities in an economical way.

There may be some evidence, parse a previous thread about increasing fuel efficiency, that buying fuel from a colder garage fuel tank, ie on a cold/early morning may give you the same volume of fuel but at a slightly higher density.
Theoretically possible I suppose, especially if you wee to buy from a Station where the Petroleum tanks were subject to a big ambient temperature change through the day.
I suspect you’ll make a greater saving to your fuel costs by following the normal advice:

I dimly remember another doper mentioning something about timing your fuel purchase with regard to a Tanker delivery but the details escape me, sorry!

Peter

remember to invert the nozzle before removing to get what is in the metal tip beyond the valve.

In my state, gas pumps get biannual inspections and have to be accurate to +/- .025 gallons per five gallons. I think pretty much all the other states have similar inspections. Of course, about 4% of the pumps fail their inspection so there’s definitely a non-negligible chance of getting one that reads high or low.

Nearly all public fuel stations are fed from in-ground tanks, and I’d guess there’s more variation in temperature from the tanker-fill temp than from ground variations.

I do sometimes look at the inspection date and try to choose stations and pumps that have been certified more recently. Probably just voodoo, but…

I can’t speak for your doper, but I’ve heard the myth that gas bought during or immediately after a fill from a tanker, will contain more silt and sediment in it, due to the turbulence in the underground tank briefly suspending silt in the gas.
A gas station attendant once assured me that there was pretty effective filtration between tank and pump, and that there wasn’t much in the way of contamination in the tanks anyways.

that final filter was a great marketing tool.

I don’t know if they consider whether the pump could have malfunctioned or was deliberately tampered with, but I know that they can issue huge fines if they find that it’s shortchanging people.

This reminds me how it’s been ages since I cared about pumping an ‘even’ amount, because I’ve been using pay-at-the-pump (credit card and/or Mobil Speedpass) for so long!

Not really. Modern digital pumps have a temperature sensor for the fuel going to the meter, and it adjusts the reported volume based on that.

Yes, but according to the seal on the pumps I’ve seen only in one direction. If it’s colder than 60 deg F it adjusts for the higher density, if it’s warmer than 60 deg F it doesn’t.