OK - week 9 you put in 60 litres at £1 per litre and don't fill up for four weeks - by the start of week 13 you have spent £60 and have an empty tank.
Me - week 9 15L at £1 = £15.00
week 10 15L at 95p = £14.25
week 11 15L at 90p = £13.50
week 12 15L at 85p = £12.75
by the start of week 13 I have spent £55.50 for the same amount of fuel.
We both have empty tanks - your error is you have factored in an extra 60 litres in your first example to take the average from whereas I will buy that extra 60 litres in four instalments on a reducing price...
Others on the forum are seeing this... and it will save them money in a reducing price market.
No Malc it's week 9 where I catch up. Let me show you:
My 1st 120ltrs cost me £138 making an average of price of 115 per ltr
Your 1st 120ltrs cost you £133.95 making an average price of 111.625 per ltr
OK so far you're ahead.
But on week 9 I buy another 60ltrs @ 100 . This means that in total I've purchased 180ltrs costing £198 which gives an average ltr price of 110
You on week 9 but your usual 15ltrs @ 100. This means that in total you've purchased 135 ltrs costing £148.95 which gives an average ltr price of 110.33.
By the end of week 12 you'll be ahead again...but as soon as I do my 60ltr fill on week 13 we're back to evens.