05-03-14, 09:19 PM
I don't think you can draw a conclusion from records over such a small mileage. There is so much inaccuracy in each "fuel-up"... i.e. it's impossible to fill to exactly the same level each time, the temperature of the fuel is slightly different etc. that the apparent mpg figure is going to bounce around and only gives a useful figure when averaged over a thousand miles or more.
My mpg chart looks similar, very wiggly... it's only when you look at the whole chart and average the wiggles out by eye that you can see there is also a slow annual rise and fall that is probably due to the oil being a bit thinner in the summer months, but servicing and tyre changes will also muddy the waters.
It's hard to see why an improvement would occur. Modern fuel injected engines with knock sensors can adjust ignition timing and fuelling maps to make use of a higher octane fuel and give more power, our old carburettor equipped engines can't. It's possible that the better detergents in the premium fuel may clean the upper cylinder, giving slightly better combustion... in which case any benefit would persist for many tankfuls of ordinary unleaded.