Anybody here see a discrepency between the mileage that your Prius tells you you're getting and what your own "miles driven - divided by - gallons purchased" calculations show? Over six months of meticulous record keeping, I came up with 44.2 average MPG on my Prius. The car, meanwhile, said 47.5. Pretty big difference.

I have some variations myself, but only about 1.5-2mpg on either side.

Alex Strand- Realtor (Dallas Area)
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I have a TCH, not a Prius, but I can offer some light on this...

Some gas stations actually pump much less fuel than the pump claims. The gas station near my house was short-pumping by more than 15 percent last winter. It was closed in February after a state inspector tested the pumps. The gas pumps were sealed and locked by the state so that they could be used at all. Those pumps have since been removed, and they are in the process of installing brand new pumps.

At some gas stations, my hand calculations agree with the mpg figure from the nav system to within just a few percent. Because of the fuel cell in these cars, the fill level actually varies a little each time, so some variation is to be expected. If one particular pump disagrees with the car for two or three tanks in a row, in the same direction, that is cause for concern.

Some gas station operators adjust their pumps to make more money. With old mechanical pumps, the calibration adjustment was a screw inside the unit, and the state could put a seal over that screw to prevent changes. With the new electronic pumps, the calibration is in software. With those pumps, it is possible for an operator to adjust the calibration without being detected, until a state inspector shows up unannounced with a calibrated 5-gallon gas can.

In Florida, the state is supposed to test every pump every year. They don't, because they just don't have enough people, or maybe because the people they do have work very slowly. In any case, they actually test pumps after complaints are filed by consumers.

If you have reason to believe that a station is short-pumping, you can complain to your state department of weights and measures. It might take some looking to find the right department contact information in your state. Every state has someone who is in charge of testing gas pumps. In some states, the contact information for that department is on a sticker on every gas pump.

Interesting thought Steve.

When doing gas studies, I carry a 1 gallon can to add gas when I run out. But it occurs to me that a 2.5 gallon or later container would work just as well. Actually some of the lawn care crews might be the best for detecting 'short' pumps.

In Alabama about 10 years ago, someone rigged up the pump so it would add the extra only at certain values. I don't know the details but I suspect they were trying to fool the standards measurer.

What does the standards measurer do with the 5 gallons after the measurement. Do they just dump it back into the storage tank?

Bob Wilson

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I also get about 2 mpg less than the screen says.

Paula

I have as much as a 6 g difference between the display and the calculated. Sometimes they are in agreement, sometimes a 1 g diff, and sometimes as much as 6 g. The shading is towards the display, i.e., the display is usually greater than the calculated.

I have never been present to watch them do it. The state officers job is to test the pump, but the fuel still belongs to the station. I would expect them to either pay for it or put it back into the gas stations storage tank. Knowing how the state likes to pay for things, I can't imagine them doing that. So it seems like they should pour it back into the storage tank.

The state code actually spells out the testing procedure. They use a calibrated fuel can with a narrow neck and a glass vial on the neck, so they can see exactly how much fuel is in the can. And they use a calibrated thermometer to do temperature compensation. But it doesn't actually say what to do with the fuel after the test is finished.

That would make sense. An unscrupulous station operator, knowing that the state code requires the tester to pump exactly 5 gallons, could program the pumps to measure the first 5 gallons accurately, and then short-pump the gallons above 5. I expect that would take more technical expertise than to just short-pump all of the gallons, but I'm sure it would be possible.

The car's ODO is likely calibrated for ONLY medium wear (mid-life) tires, EXACTLY correct "warmed up" tire air pressure and temperature.

Basically too many unknowns for any reasonable level of accuracy.

Easy way to beat a speeding citation based on the trooper's speedo.

In Florida, accuracy of fuel pumps is regulated by the
Florida Department of Agriculture and Consumer Services,
Division of Standards, Bureau of Petroleum Inspection.

Contact Information:
Telephone: (850) 488-9740
Fax : (850) 922-6655
Field Office Telephone: (850) 487-2634
Postal Address:
Bureau of Petroleum Inspection
3125 Conner Blvd.
Lab 1, Mail Stop L29
Tallahassee, FL 32399-1650
Web site:
http://www.doacs.state.fl.us/standar...o/contact.html

Every state has something similar.