Andy, not looking to scupper your experiment but Honest John reckons it takes 3 tank loads of fuel before any changes kick in. The chances of the Shell fuel impacting on your journey are probably quite limited. I suspect to do it properly you would need to do the journey there and back with supermarket fuel, move across to Shell and then do it again purely with Shell after running Shell for a bit.
My own view is that using regular Shell against regular supermarket fuel will give marginal gains but the difference will only work for you if the price difference is slight. If you pass a supermarket garage on the way home and it is 3p a litre cheaper then stick with it. If you pass both and there is 1p in it then go with Shell. Reading your post again, that fits with what you said. Well done that man.
Good fun to do though, for all us mpg efficiency nerds.
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I used to have an S40, V60 and XC60 so I am allowed here, honest.
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