New data on petrol prices across Sydney has "blown away" the myth that supermarket shopping dockets and big service stations give better value at the bowser.

Instead, a Speedway franchise at Granville (cheapest E10 and regular unleaded), an independent at Milperra (cheapest premium 95) and the Metro Fuel at Belmore (cheapest premium 98), have emerged as Sydney's best value service stations.

The most expensive? Coles Express at Kingsgrove (E10, premium 95 and premium 98) and 7-Eleven in Mosman (regular unleaded).

More than 2000 service stations in NSW must provide information to the Fair Trading website FuelCheck every time they change the price board, giving the first real-time data on prices.