Motorists are being cautioned by the Automobile Association not to assume biofuel is better value than petrol, even though it is cheaper at the pump.
The organisation, which claims more than a million members, says in its latest Directions magazine that a test driver travelled almost 8 per cent further in a 2-litre car on pure petrol than on a 10 per cent bioethanol blend from renewable dairy wastes.
That was more than a 3 per cent differential, which leading biofuel supplier Gull says is the accepted industry figure from international research.
It says it compensates motorists for the performance difference by charging less for the biofuel.
AA spokesman Mark Stockdale said that although the biofuel used in the test about two months ago was bought from Gull for 5c a litre less than pure petrol, it would have cost 10c a litre more to travel the same distance in the test car - a 2007 Honda Civic.
The car travelled 205km on 15 litres of petrol, and 190km on biofuel, on the same day and same route.
Mr Stockdale said international commodity prices for bioethanol, which were higher than for fossil fuels but mitigated in New Zealand by a tax exemption, suggested a 10 per cent blend should be sold for about 6c a litre less than petrol.
Gull was selling its 91-octane biofuel blend in Auckland yesterday for 4c less - at 199.9c a litre - and its 98-octane was 8c cheaper than pure petrol from the main oil companies.
That follows a general 4c industry rise on Friday.
But the Australian-owned company joined the Bioenergy Association in accusing the AA of running an unscientific experiment by comparing only two 15-litre tanks of fuel and of undermining efforts to provide motorists with lower-carbon energy supplies.
"When you do a one-off 15-litre test and you get to an 8 per cent difference, logic should dictate you ask some questions and you don't go ahead and publish it," said Gull NZ general manager Dave Bodger.
"All the worldwide information talks about 3 per cent less energy on a 10 per cent blend."
"There's no secret that an ethanol blend generally has about 3 per cent less energy in it but you've also got more oxygen for a better burn."
Mr Bodger said there could be wide variability between different batches of fuel, which was why it was dangerous to rely on such a small sample.
Bioenergy Association executive director Brian Cox said the AA was being irresponsible, and failing to provide important details about the test, such as where it bought its fuel, how it was transported and the history of the test vehicle.
Convenience beats cost for biofuel buff
Like many motorists, Jeff Cummings fills up where it is most convenient - whether with pure petrol or biofuel.
Price comes second unless there is much difference between rival outlets and, because he works near Gull's Kingland service station, that makes him a regular customer of its 10 per cent bioethanol blend.
Although the public health addictions counsellor was a bit disappointed yesterday to hear that cheaper fuel may not end up costing less, he would need a lot more information before changing his buying habits.
He was surprised to hear of the AA's comparison between the staying power of pure petrol and biofuel, but said he looked forward to more tests.
Mr Cummings said he remained sceptical about global warming and, although he believed in looking after the planet, lower carbon emissions were not why he filled up with biofuel.
Although a fan of petrol-guzzling performance vehicles in his youth, his response to high fuel prices now he was a family man was to drive a car with more modest needs - usually a 1.3-litre Ford Laser, although that is being repaired after being crashed into.