Are fuel companies trying to shore up their profits by reducing the octane rating?
Q. Fuel costs may have gone down but so has the efficiency of my Hyundai Santa Fe V6. I used to expect to get about 500 miles out of a tank of petrol; now my trip computer tells me to expect 420. Are fuel companies trying to shore up their profits by reducing the octane rating?
A. Petrol sold in UK filling stations must conform to the relevant British standards, which ensure the energy density (the amount of energy per litre of fuel) stays constant to within about 1%. The specification also requires an octane rating — a measure of the fuel's resistance to spontaneous detonation, or pinking — of 95 or above.
Instead it could be that your car's trip computer is causing the confusion. Most of these devices base the range calculation on the average miles per gallon over the past few tens of miles. If you have changed the day on which you fill up, or are driving on new routes, it might be that your last few hours before filling up are spent burning fuel inefficiently — by sitting in traffic jams or picking your way through a busy town, for example. The result will be a reduced range estimate.
Calculate your true range by using the car's mileometer to measure the number of miles you travel on one tank of fuel.
There are mechanical reasons why the fuel efficiency might have dropped too. The current cold weather, for instance, makes the engine less efficient — it takes longer to warm up — and means that fuel is used to heat things such as the seats, windscreen and cabin.
And if it's been a while since you last had the vehicle serviced, book it in: declining fuel economy might be a sign of underlying engine trouble.
Tim Shallcross used to train AA patrols to fix cars. Now he advises the Institute of Advanced Motoring – read more from Tim here.