When it comes to oil prices, what goes up doesn't necessarily come down - or at least not by as much.
Local retail price for diesel jumps when international oil prices rise, but don't fall as much when global prices fall, the Consumer Council found.
The research, launched after a group of drivers complained in March last year about what they said were unfair pricing practices by oil companies, compared local diesel prices with the Brent crude oil price from January 2007 to July this year.
It found the drivers were right up to a point: the gap between the import price and retail price of diesel widened when crude oil prices fluctuated.
Based on limited information, the council said the net profit earned by some companies - which it refused to name - had increased from HK$0.03 per litre to HK$0.83 over the past five years.
The oil firms said they were not earning as much as estimated by the council due to rising costs and discounts offered to professional fleets.
But the council said they should release more information to show this was the case.