Sorry have to disagree having been involved in motorsport and testing for 30 years.
We are not talking about small changes in economy we are looking at 10% which is significant.
Temperature does play a part but can be compensated for in the overall calculations if you get the control runs correct.
My driving over this period will be very similar although not identical (obviously) but 2 months is a reasonable time to make a judgement on a device or change. I do the same 80 mile commute every day of the week and don't use the car at weekends (thats pretty consistant)
Supermarket fuels do not contain the same additive packages as premium brands such as Shell or BP, I used to worrk for the company that supplied them. Branded fuels will always be kinder to an engine. If you are in doubt of that take a look on Scoobynet. One guy thought he was clever saying there was no difference always ran it on supermarket fuel. Car failed at 23,000 miles theres photos on there with the head off you want to see the carbon and on a car that was driven hard on mainly motorway miles. I don't need no convincing supermarket fuels are designed to comply to the minimum BS rating and thats it, thats why they are cheap.
If you have a diesel try Millers Diesel Power Sport 4 and then come back and tell me it does not make a difference!
We are also using some very sophisticated technology with dyno testing and dynamic road testing to determine Power, Torque and MPG. These machines don't lie.
Sorry you dont think its possible but as an ex works and manufacturers test driver let me tell you it is, how do you think cars get developed and technology advances?
