As oil prices plunged last week, consumers may have noticed a different trend at the pumps.

Despite crude oil falling below $55 a barrel, gas prices actually went up, BMO Senior Economist Benjamin Reitzes noted in a research publication.

"I couldn't help but notice how gasoline prices had ticked higher from the previous week," Reitzes wrote. "With Brent crude falling below $55, a level last seen in 2008, gasoline around one dollar per litre seemed odd."

Reitzes looked into the numbers further, comparing the current prices to a chart which tracked gas and oil prices since 2008.

In earlier years, the trend lines show the price of gas (in red) mirroring the rise and fall of Brent crude oil prices (in blue).

But more recently, the lines diverge.

"The chart suggests that gas prices should be well below one dollar," Reitzes said in his note.

BMO chart of gas and oil prices

In the United States, gas dropped to some of its lowest levels since 2009 on Saturday, hovering around the US$2-a-gallon-mark.

But in Canada, Reitzes said, the low oil prices aren't significantly affecting the cost at the pumps.

"Simply, consumers don't appear to be reaping the full benefit of lower oil prices," he said.