I am surprised your fiancé noticed a difference in milage as the price increase equates to a 2% (approx) reduction in fuel.
If her car does 35MPG then from 111.9/Litre equates to 68.85 Miles, the price increase would reduce that to 67.62, so its not noticeable.
There are many other factors in play here, are her tires correctly inflated and aligned? Also in UK your weather is getting colder, so the car will use more petrol, especially on short journeys (see below).
Is there more traffic on the roads, are there roadworks slowing or stopping traffic flow?
Whilst I see your point at your fiancés inability to understand she had got less fuel for her money, don’t you think your reaction was equally as stupid, as the reduction in fuel was so small as to be negligible, and you took no account of other variables that may affect fuel performance?
Fuel facts
To help drivers understand better the impact of winter driving on their fuel bills, the AA Fuel for Thought campaign offers the following facts:
- The heated rear window uses around two per cent extra fuel. If both the front and rear heated screens are used the increase in fuel consumption equates to six per cent as heated front screens consume more power than the rear screen.
- Even at a relatively mild outside temperature of +10C, a car's fuel consumption for the first mile will be around 40 per cent higher than with a warmed-up engine. This falls to around 16 per cent over the next three miles and, even up to 6.5 miles from start-up, fuel consumption can be 8 per cent higher.
- The increase in fuel consumption due to cold weather is similar for both diesel and petrol cars. AA tests show extra fuel use after 1 mile and 3 miles respectively are:
- typical small petrol car – 34% and 17%
- medium petrol car – 38% and 16%
- small diesel car – 32% and 12%
- medium diesel car – 43% and 18%
- For demisting windows and getting the cabin comfortable, basic air conditioning on a small car can increase fuel consumption by up to 10 per cent. A more powerful petrol engine will suffer less, as will a larger diesel.
- Getting stuck in a traffic jam is another daily problem with winter driving. Even with a warmed-up engine, a petrol car can lose at least two-thirds to three-quarters of a litre of fuel every hour, or 1.2 to 1.4p a minute. A diesel car can lose a third to half a litre of fuel an hour, or 0.6 to 1.0p a minute.