In message , at 13:04:05
on Mon, 16 Apr 2012, remarked:
I made many LU journeys in Z1-4 and one costing 1.40 to Z5.
I was charged 14.40, rather than 12.00 - the Z1-4 cap + 1.40.

...

Zone 1-4 + an extra single journey is how I assumed the worked too, but
I'm apparently mistaken. Does TfL explain exactly how it works
anywhere?

This page, http://www.tfl.gov.uk/tickets/14837.aspx , states

"Oyster will work out the cheapest combination of fares for all
your journeys in one day."

A zone 1-4 travelcard plus a single oyster fare would have been the
cheapest alternative.

The Z1-4 cap is £10.60, plus £1.40 is £12.00

But the Z1-5 cap is £15.80 - so how does £14.40 arise? (Other than
being £15.80 minus £1.40, which makes no sense).

I assumed £14.40 to be the sum of Oyster single fares for all his
journeys that day.

I see. So he made £14.40-£1.40=£13.00 worth of trips in Z1-4, which
would have been capped at £10.60, had he not strayed into Z5. Makes sense.

FSVO "sense".

"It adds up", if you prefer that as a description.

If I understand the theory behind the sums above, we can re-write the
Oyster "Price Promise" so rather than what it says above, the wording is
more like:

"Oyster will cap all your journeys in one day to the cost of the
relevant Travelcard that would have been sufficient and necessary to
carry out all those journeys. (There may be other combinations of
ticket which would have cost you less.)"
--
Roland Perry