In 2013, Gull lost a tax case in respect of their use of blending butane into petrol to reduce the excise. Butane had an excise duty of 10 cents a litre and petrol was at 48 cents a litre.
They were blending in relatively small amounts in but attempting to save the 38 cents per litre in excise duty which saved them $10m in excise.
Customs disagreed and the Supreme Court upheld the Customs interpretation which meant Gull incurred $13m in penalties and interest for a total bill of $23m.
source:
http://www.stuff.co.nz/business/indu...trol-tax-fight
At the time, expert evidence was that the blended spirit wasn't chemically identical to petrol. Butane has an energy density of about 7,700 Wh per litre and petrol is about 9,500 Wh per litre (for reference ethanol is 6,666.6 Wh per litre) so it would not have the same 'energy density.' E10 roughly has 9,200 W-h litre so it can have a higher research octane number (RON) but have less density so relatively the savings from using it are offset by the increased fuel burn.
Disclaimer - I don't know if the blending is still occurring or indeed what brew is live. I'd also say that actually getting reliable and accurate relative fuel economy comparison out of one tank of gas isn't actually achievable unless you have an identical car following you with the other gas, you do a particular long distance run with little traffic and similar driving styles to judge it (or you have the sort of data acquisition and logging you have on a race car) or alternatively do the test under controlled conditions like on a track - believe me I've spent a bucket load of time looking at it in the context of endurance racing - even a variable like tyre pressure, rolling resistance of different tyres, ambient temperature can all play a part (actually it's mainly the driver that makes most of the difference).
In terms of savings from the fuel discount programmes, by judicious and disciplined use of stacking in the Flybuys Z and Caltex Pumped scheme (e.g. buying in $40 increments and stacking), I have achieved an average discount per litre of around 22-23 cents per litre off pump price for all the petrol we have purchased since last August that's equated to about $550 off the pump price. That is across two cars but it represents about 4 times the discount most folk could access but don't because they don't know how it works or don't perceive that there is value in doing so - also $550 after tax is about $820 income pre-tax which for a lot of folk isn't small stuff - even half of that for most folk (say one car) isn't small stuff.