In one instance, a small hybrid SUV exceeded its advertised fuel consumption by 27 per cent, consuming more than a litre extra every 100 kilometres.
The Australian Automobile Association issued the warning on Thursday after testing more vehicles in real-world conditions and comparing fuel consumption and range to their laboratory tests.
The findings come amid fuel shortages and rising prices caused by conflict in the Middle East, as well as record-breaking sales of low-emission vehicles.
The motoring body analysed 17 vehicles in its latest tests, driving the cars along a 93km course around Geelong in Victoria, on urban and rural roads as well as motorways.
Seven vehicles exceeded their advertised energy consumption by more than 10 per cent, with the GWM Jolion Hybrid burning 27 per cent more petrol than its lab test result.
Two other hybrids also consumed much more petrol than expected, with the Audi Q5 mild hybrid and GWM Haval H6 Hybrid both using 19 per cent more fuel.
Petrol cars including the small Kia 4 and the Honda ZR-V medium-sized SUV consumed more petrol than advertised, by 15 and 10 per cent, respectively, and two BYD electric vehicles fell far short of their promised range.
The BYD Dolphin electric hatchback covered 313km rather than the 410km promised — 24 per cent less than advertised — and the BYD Sealion 7 SUV's range was estimated at 469km rather than the 576km on its sticker.
The results proved consumers should scrutinise promises made by automakers, association managing director Michael Bradley said, and seek independent information before making a purchase.
"Australian car buyers are right to be deeply sceptical of the fuel consumption information car makers are providing," he said.
"It's critical that Australian consumers know exactly how much fuel a car will actually use, how much they will cost to run or how far they will go on a single charge."
Two of the 17 cars tested did post better than expected fuel consumption, with the Subaru Forester AWD using one per cent less petrol than its lab result and the Chery Tiggo 8 Pro using two per cent less.
More than three in four petrol, diesel and hybrid vehicles examined in the group's Real-World Testing Program have failed to meet their lab test results, however, and none of the electric cars analysed met their estimates.
The vehicle-testing program, which launched in 2023, was introduced after a 2015 Volkswagen scandal in which it was found using software to alter vehicle emissions during laboratory tests.
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