MOTORISTS' wallets are being drained at the bowser by petrol stations' failure to pass on price cuts.

The average international wholesale unleaded petrol price on July 22 was $1.17 a litre, down from $1.22 in April, the Herald Sun reports.

But independent petrol monitoring service Fueltrac says that in the three months to June 30, drivers paid an average of $1.27-$1.30.

Fueltrac monitored transactions at petrol stations in about 400 Melbourne suburbs.

Spokesman Geoff Trotter said motorists should have benefited as the world price fell.

"There was a significant drop in international prices to July, and we are not seeing those international price drops in retail pump prices. That is, retailers are increasing their margins," he said.

"It's actually even worse for diesel motorists. They are keeping the diesel prices up really high."

According to Fueltrac's statistics, Berwick, Essendon, Avondale Heights, Strathmore Heights and Lower Plenty were the most expensive suburbs, all averaging above $1.31.

Springvale and Elwood were the cheapest, averaging closer to $1.24.

Mr Trotter said a lack of competition in some suburbs pushed prices up.

Other suburbs reaped the benefits of hosting large independent service stations, such as United, Liberty and 7-Eleven.

Oil industry expert David Cumming said prices at some service stations had dropped as low as $1.18 at the weekend, and the large swing between the top and the bottom of the price cycle pushed up the average pump price in Fueltrac's data.

"There is a 12 to 14 cent swing in weekly petrol prices, and it's because they go up to these ridiculously high prices that we get a much higher average," he said.

"But we know 60 per cent of Victorian drivers purchase fuel on the three cheapest days of the weeks."

Mr Cumming also said changes to the cycle had confused motorists.

"The lowest petrol prices are now on a Saturday. It had been a Thursday for many years, and before that a Tuesday," he said.

He said news that Mobil had sold its 302 Mobil-controlled retail sites to 7-Eleven may have changed the petrol price cycle.

"It will be interesting to see how 7-Eleven go once they take over. Will they be willing to play the same game, and jump up 14 or 15 cents within a week?" he said.

Read more on the Herald Sun.