Australian drivers are likely paying hundreds more in petrol each year than advertised on the windscreen sticker.

The advertised fuel economy for vehicles are the results of tests in a laboratory overseas, and not their performance in real conditions on Australian roads.

But Australians only have the lab results to consider when comparing the fuel economy of different cars.

When 30 new cars were tested on Australian roads in 2017, nearly all of them performed worse, and some of them much worse than advertised.

According to the Australian Automobile Association's (AAA) results, cars that on their sticker supposedly consumed about 8.5L per 100 kilometres actually consumed about 10.8L on average over that distance.

That means if someone filled up their tank at $2 a litre and went for a drive from Sydney to Canberra using one of those cars, the cost of that trip was not about $49, but in fact $62.

Added up over a year the average Australian driver would end up spending hundreds of dollars more on petrol than expected.

But the real problem according to the AAA is the variation in how much worse each car is compared to the results on their sticker.

Fuel consumption stickers note performance will differ in real-world conditions — but by how much varies greatly from car to car.

While three of the 30 cars they tested performed in line with their advertised fuel economy, the average car consumed 23 per cent more fuel than their sticker result, and in the most extreme case one diesel SUV performed 59 per cent worse than its official result.

So, when people go to buy a new car, it can be almost impossible to know which is actually the most fuel efficient — or which has just been engineered to do well in a lab test.

200 cars to be tested on Geelong roads

In response, the federal government has launched a "real-world" testing program, fulfilling one of its election commitments.

Over the next four years, AAA will test 200 different cars, utes and electric vehicles on Australian roads to see how well their fuel consumption and CO2 emissions match up to their lab results.

In a statement, AAA managing director Michael Bradley said the scheme will save consumers money and benefit the environment.

"Australian car buyers have for too long been misled regarding their vehicle's fuel consumption and environmental performance," Mr Bradley said. 

"This program will deliver Australians truth-in-advertising and drive down demand for cars that over-promise and under-deliver."

As part of the program, a new website will allow people to compare the on-road results of those cars when they're looking for a new car to buy.

The AAA has already begun conducting the new tests from Geelong in Victoria, where cars are fitted with a device and then sent along urban roads in town, along the motorway and finally hitting rural back roads, driving to the rules in real traffic.  

It expects to publish the first results from the middle of next month.

The federal government has committed $14 million to fund the four-year program.