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Thursday, August 10, 2006



Dear Friends:



Have you ever pumped 15 ½ gallons of gasoline into a 15-gallon tank? Truth is, the tank capacity in your manual is an educated guess.

The National Institute of Standards and Technology (NIST) says that actual gas tank capacity can vary by as much as 3 percent. On an 18-gallon tank, that’s a half-gallon of gasoline.

This illustration shows a typical fuel tank. If you ignore the automatic shut-off function on a gas pump and keep fueling, you can end up with gasoline in the vapor headspace or the filler pipe. Fuel may also enter these spaces if the gas station is on a slight incline.



The bottom of a gas tank is also out of reach for the fuel line. None of this is included in the fuel tank capacity rating.



Some motorists rely on their fuel gauge to determine tank capacity. This too is misleading. The gauge is designed to warn you before your car runs out of fuel. How useful would it be if it didn’t read “empty” until you were totally out of gas?



Finally, even those trip computers in newer vehicles can be misleading. They measure a number of variables, including road terrain, traffic and weather. If you were driving north on I-17 and began climbing through the mountains south of Prescott, you’d see a big difference in the computer readings.



Still, if you think you didn’t get what you paid for, please contact the Arizona Department of Weights and Measures by calling (602) 255-5211 or filing a complaint online at http://www.azdwm.gov/complaints.htm. They will look into your complaint.



As always, I encourage you to visit my Fuel Facts Web page devoted to gasoline information at www.azgovernor.gov/fuelfacts. Please check the site often throughout the summer as we add fuel saving tips.



Yours very truly,



Janet Napolitano

Governor

 

I have been running a test to see if I fill up again within 10 miles, I can get better agreement between the calculate tank MPG and the display MPG. This isn't working but it is not uncommon to add nearly an extra gallon on the second top-off. Again, to the first click.

Bob Wilson

 

Be careful with super-fills on gasoline engines. The fuel vapour recovery system can be damaged if you get liquid fuel into it.

As a side rant, how many filling stations actually have workable filler hose flaps that properly and completely seal off the filler neck from the atmosphere during gasoline refuelling, so the HC-laden air in the tank is displaced into the activated charcoal canister? None that I've ever seen. So although the tailpipe emissions of some gasoline engines are very clean in a relative sense, when refuelling, that is all undone (and then some) by all the HC fumes that are pumped into the air at the fueling station.....a volume of HC contaminated air equal to the volume of the tank in many cases.

On the other hand, because diesel engines don't use a volatile fuel like gasoline, a fuel vapour recovery system is unnecessary, and so SuperFills (TM) way beyond the first click of the nozzle are possible. This has been demonstrated with the smart fortwo cdi in Canada numerous times. A SuperFill (TM) is where you minimise the air bubbles inside the tank by lifting the car on one corner (usually the RR one on a smart) with a jack while at the pump island, and fill 'er up to the absolute brim. This can cause gas pump rage ;-)

Most recently, www.clubsmartcar.ca member |Eddie|Eddie|Eddie| SuperFilled (TM) smart fortwo cdi's "22 litre" tank with something between 27 and 28 litres, and drove from Kelowna BC to Edmonton AB in a roundabout route that amounted to 965 km (600 miles) before the tank risked running totally dry (dangerous/fatal for the high pressure fuel pump in a common rail engine).

It took 27 L to refill this nominal 22 L tank, using the same SuperFill (TM) technique, with a fuel consumption of 2.8 L/100 km or 84 US MPG. The speed on the highway was 52-55 MPH, 44 psi in the tires.

My conclusion? I always manually hold the flap on the fuel nozzle tightly against the fuel orifice of my gasoline car when refueling, to treat the vapours properly, and I don't usually go past the first click. In my smart fortwo cdi cabriolet, I don't SuperFill (TM) but I do top off, usually getting 24 L in my 22 L tank.

 

That is why I stop at the first click.

 

I usually fill to the first click, though if the number of gallons pumped is somewhat lower than normal, perhaps due to swirling in the top of the tank causing fuel to "splash" the nozzle prematurely, I will pump until the second click off (another fraction of a gallon).

I usually run just past when my display shows "0" bars, and I nearly always fill 12.4 gallons +/- .1

The only time I've filled over the rated capacity (I filled 13.4 gallons) was after a long climb over the grapevine (several miles climbing at 6% grade). During the climb, the tank read that I still had two bars of fuel, level, but as soon as I began the descent, this dropped to zero, and I more or less coasted to the next station. I imagine driving for so long on that incline tilted the tank and caused fuel that is usually in the "unusable" region along the bottom of the tank shown in the diagram to be consumed very rapidly.

 

It doesn't work like that anymore, at least not over here. Instead there is an air pump sucking in air near the top of the nozzle. The fumes are then brought back to the gas container. Some fumes can certainly escape, but most is captured. The old gas pumps without this kind of recirculation are banned.

From what I've understood, this recirculation more or less pays for itself, since you pay also for the fumes that are recirculated. The gas station gets paid twice for them.

 

Europe is light-years ahead of North America in environmental regulations (after being light-years behind from 1969-86).

I have never seen one of these vacuum gasoline pump nozzles at any of the hundreds of fuel stations I've been to in North America. We should have them, but don't, to my knowledge (maybe in southern California?).

All the ones I've seen consist of a flap of coloured vinyl (usually torn) which would not seal against any car's fuel filler neck. And so all the gasoline vapours are discherged to the atmosphere when refueling.

 

If you're talking about the kind with the big rubber sleeve over the nozzle that's supposed to fit tight against the filler neck on the car, they're certainly not common in Virginia. In fact, I can't think of the last time I saw one here. The vast majority of stations have the old fashioned kind with a bare steel nozzle. I cringe at all the fumes that are escaping to the air every time I fill up. Not to mention the dopes who stand there and smoke while filling up their pickem up trucks (with the engine running).

B

 

Empirical evidence on my '06, Prius is that it is greater than 10 gal. The manual, as I recall cites an 11 gal. capacity. I will drive well into the blinking last square. That is greater than 10 gal.

In any case, I stopped topping off when California put the vapor recovery nozzels on the gas pump.

Finally, if there's no certainity in the size of the tank in general, I think topping off only adds to the level of uncertainity when calculating mpg.

Incidently, I think I saw that smart car on Vancouver, Is (around the Malahat Inn)

 

It could be my smart car that you saw on the Malahat, or it could have been one of the 749 others that have been sold here on Vancouver Island over the past year and a half! I drive past the Malahat Mountain Inn every couple of weeks....