A regular reader asks about fuel consumption and whether a full tank has any bearing on a vehicle’s fuel economy. The reader also asks if manufacturers provide their claimed fuel consumption figures based on the vehicle having a full tank.
This question is best answered by saying that claimed fuel consumption figures from manufacturers are generated in an environment completely different from those which any driver will experience on a daily basis, or will experience in a lifetime of vehicle ownership.
Manufacturers’ fuel efficiency engine testing is carried out in laboratories under controlled conditions and temperatures and includes short spells of cold start, acceleration, deceleration, steady speed and idle running. The claimed urban (around town), extra urban (higher speeds) and combined claimed fuel consumption figures are calculated from the results gained. My understanding is: in terms of total distance travelled, the testing is completed in less than 20km of equivalent driving.
So no head or side winds, no extra loads, no accessories switched on and no changes in terrain to deal with. It’s all done in-house. I don’t know about fuel levels but I suspect it wouldn’t make a great deal of difference.
Some people say to achieve the most savings at the pump it’s best to add fuel under cooler conditions such as early mornings or evenings, as the fuel is more dense at that time and in theory you gain in volume. The other argument is: the more fuel added the more weight that has to be carried, which can negatively affect overall consumption.
So there are lots of theories and points of discussion on this topic for those who are interested in drilling down into the nitty-gritty. For the mainstream buyer and user it’s simply best to use the manufacturer’s claimed fuel consumption figures as a guide only when comparing similar size cars fitted with similar size engines. All the testing is done under the same conditions so you are comparing apples with apples. And it’s still a good guide to use because there are definitely some manufacturers who are achieving better claimed combined fuel consumption figures than competitors.
I have seen manufacturer’s claimed fuel consumption figures beaten consistently in real world conditions. But this has always been in a fiercely competitive environment such as the AA Energywise rallies I was involved in organising. Driver concentration was focused on throttle and brake control among other things to achieve amazing fuel consumption figures. And competitors were expected to keep to the legal speed limits during the rally and risked receiving penalties for failing to reach each stage finish in a certain time to help stop saving fuel by driving far too slowly.
So events such as the AA Energywise Rally do highlight the fact that the manufacturers’ claimed figures are possible to achieve in certain conditions. But once these sorts of events are over, the drivers and crew who took part are far more likely to revert to a less fuel-conscious driving style.
For the average driver then, the manufacturer’s claimed figures are highly unlikely to be reached or exceeded especially in consistent and regular stop-start conditions.
It’s in this environment that hybrids shine and have a much better chance of achieving close to the official figures as vehicles are often running on their electric motors only, plus they have the added benefit of producing zero tailpipe emissions.
The other important point to remember is that when claimed fuel figures are produced the engine is new. And when vehicles are entered into fuel economy rallies they are in perfect tune and the other important steps such as having the tyre pressures set correctly are all taken care of. Fuel consumption will suffer for reasons such as driving style, under-inflated tyres and lack of servicing.
Whether the fuel tank is full is never going to be a major factor with a mainstream automobile. So there seems little point or benefit in keeping the tank filled to the brim if the weekly commute doesn’t warrant it.