miles per gallon difference between different fuels

[fluffyspit]

Hey all, can anyone tell me why it is, if i fill up my car with unleaded petrol at say' safeways' i only get 34/35 miles to the gallon, but if i fill up at 'Jet' i get 39/40, is the petrol mixed differently, do safeways water it down???? ha ha please let me know as it is very perplexing, Ta xxxxxx

[osullivp]

probably more to do with the type of driving you do than the actual petrol, unless there's a complicated chemistry based answer. (i'm sure you knew stop start driving uses more petrol than a steady 50mph, i just thought i'd insult your intelligence)

[moonacre]

Petrol has something called an octane rating. In effect this measures how flammable or explosive it is. The higher it is the easier it burns. Higher octane fuel is more expensive to produce but can give better fuel economy. The octane rating you will see at the pump just shows the minimum octane rating of the fuel being dispensed.

Supermarkets selling cheap fuel will always be offering the lowest octane they can. Other garages may find it more efficient to fill their tanks with higher octane fuel for various reasons - I seem to remember that at one time the average octane level at Shell garages was usually higher than (the minimum) indicated. This is good news for the motorist.

The other main difference is that more expensive fuels often have added detergents which have been proven to increase mpg.

[fluffyspit]

Thank-you 'Moonacre'......I thought maybe I was crackers, I also posted this question on the travel section and along with the answers on here and there,nearly everyone was either stating the obvious, drive at a certain speed and style jargon, you have given me a logical reason for the difference and I will certainly try to fill up at the more expensive/higher octane one's cos it really does make a hell of a difference!!!Thanks again xxxx