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How to measure your fuel consumption

Although sales show many Canadians don't seem to care about gas consumption, it's a good idea to know the ballpark figure.
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“About $20 a week” or, “Three tankfuls a month” or even, “What do I care? My company pays for the gas.”

Talk to most motorists about their fuel consumption and the answer is usually something like the above. And at one level, why not? At the end of the day, the cost and the frequency of the fill-ups is the bottom line.

Trouble is, responses like that are completely meaningless to anyone else. How many kilometres do you drive in a week? How many litres does it take to fill your car’s tank? Was that $20 at this week’s gas price in Quebec, or last week’s gas price in Alberta?

Judging by the surging sales of large pickups and SUVs this year, it seems an awful lot of Canadians really don’t give a second thought to fuel consumption. But if you do care, you’re flying blind if you don’t know what the actual numbers are, in hard litres per 100 kilometres.

If your car is nearly new, it may be able to tell you. Trip computers that display average fuel-consumption are increasingly commonplace. That said, in my experience these displays are often only in-the-ball-park accurate. Same story with aftermarket devices, like my ScanGauge, that plugs into the on-board diagnostics ports of 1996-and-newer vehicles.

It’s really not all that hard to measure your fuel-consumption yourself. Start with a full tank and write down the odometer reading. Keep a record of all the gas you add. Refill the tank at the end, again noting the odometer reading.

Now add up all the gasoline added since the start (including the final fill-up, but NOT including the gas added for the initial fill-up), divide the total litres by the number of kilometres driven, and multiply the result by 100.

(Still living in the Sixties? Then convert to miles-per-gallon by dividing the L/100-km figure into 282.5, which equals 49.9 mpg. American? Divide L/100 into 235.5, which equals 41.6 mpg US).

Aside from actually doing the math, the biggest challenge is to achieve the same, repeatable level of “fullness” at the start and finish of every test.

In theory, your best bet is to fill the tank right up to the top of the filler neck each time. In practice, however, modern fuel tanks have complex plumbing designed to prevent gasoline vapour from escaping into the atmosphere. As a result, it is inadvisable – not to mention time-consuming—to brim-fill the tank that way.

Unfortunately, the alternative—filling the tank until the gas pump automatically shuts off—is potentially inconsistent. Not every gas pump shuts off at exactly the same point; as well, the shut-off point can vary depending on the position and the angle of the pump nozzle in the filler neck.

To minimize these potential inaccuracies, try to do your initial and final brims at the same pump at the same gas station, and with the nozzle in the same position, each time.

If you’re really obsessive, there are other wrinkles. For example, I check every test car’s trip odometer over a known distance. Variations of one or two percent are commonplace, and I’ve even encountered errors as high as five percent. Your own car’s accuracy will change over time as the tires wear down, and will likely change again when you replace worn tires with new ones.

Then there’s the way gasoline expands or contracts depending on ambient temperature. I make a point of almost-filling the tank the day before I do an “official” fill-up, so the gasoline has a chance to stabilize at ambient temperature.

Of course, you can’t control everything. We have to take it on faith that the pumps at the gas stations are honest and truthful. But with a bit of care you can get close enough.

Once you have a good idea of what your fuel consumption actually is, you have a benchmark to determine whether it’s as good as it should be.

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