Making sense of fuel consumption claims

By Martin Pretorius

 It's a question which, in some variation or other, regularly pops up on our “Ask AutoTrader” section: “My car doesn't return anywhere near the claimed fuel consumption figures. What's going on?” These are not isolated incidents either, in fact, it's something we experience with our test cars all the time. Deviations of up to 30 percent are commonplace, and we usually just ascribe this to some lead-footed driving or heavy traffic – but that's not the only reason for the discrepancy between the manufacturer's claims and real-world results.

 Sometimes, it really is the car's fault

If your car isn't in perfect mechanical nick, it most likely won't perform according to its design specifications. All services have to be up to date and according to manufacturer's specifications. Seemingly unrelated issues such as worn wheel bearings or overfilling with engine oil can have quite a dramatic impact on your fuel consumption, along with other factors such as running the air conditioning all the time, or fitting those fat tyres to “score some street-cred”. That heavy aftermarket bumper on your city-bound SUV? There's some fuel-draining extra aerodynamic drag right there. Anything which goes outside the manufacturer's design specifications will have an impact on your consumption, and anything which puts extra load on the engine will increase its thirst. Also have another look at your driving habits, as explained in this article <insert link to other consumption article>

 But my car is still new and original?

Even brand new cars still generally don't match their consumption claims, barring a handful of exceptions (Suzuki and the smaller-engined Volkswagens come to mind). It's quite a let-down to see your new car underperform in this critical area, and the disappointment only grows when the salesperson assures you that this is in fact normal – or that “the vehicle has to be run-in first, then it will improve”. Sure, it will most likely get better when the car has a few thousand kilometres under its belt, as even modern cars need to “wear-in” their mechanical bits. But even then, the chances of you even approaching the claimed figures are very slim indeed. The system is, quite frankly, stacked against that ever happening.

 Do the manufacturers lie to us?

This doesn't mean that the manufacturers expressly try to swindle the consumer, however. They're simply using the system to their advantage, by optimizing their cars to be as frugal as possible during the official test procedure. And indeed, if you could find a way to simulate that test procedure on your daily commute, you'd most likely match the official claims. Unfortunately, that would be impossible, simply because the tests are completely out of touch with real-world driving. It's not the manufacturers at fault here, it's the lawmakers and enforcement authorities who should take the blame.

 What's wrong with the testing procedures?

These officially-sanctioned consumption results are obtained at the same time as the emissions test results. The aim is to level the playing field by testing all vehicles according to the identical test procedures, with the data thus gathered to be applied to calculations of both carbon- and consumption taxes. Sounds good so far.... but there's a catch: these emissions testing procedures have their origin in the 1960s, and they haven't really been adapted to modern traffic or driving styles.

 The tests have become too easy

As an example of the ease with which modern cars would complete these outdated tests, consider this: the “urban” part of the test requires the vehicle to accelerate from standstill to 50 km/h in 26 seconds, and eventually cover slightly less than 4 kilometres in 13 minutes – all without the prolonged idling periods you'd encounter in typical stop-start traffic. 50 km/h in 26 seconds? It's hard to imagine taking that long to not even reach the speed limit...

The “extra-urban” component is no more realistic, because it includes (among other tests) a “sprint” from standstill to 70 km/h in 41 seconds, and an overtaking run from 70 to 100 km/h in 35 seconds using fifth gear. Neither of these tests reflect modern driving conditions, which combines with far more powerful cars to render their results rather pointless.

 Someone is fixing the problem

The current, outdated test procedure is due to be replaced by a completely new set of rules within the next year or so. The new procedure will be designed to simulate modern driving patterns much more closely, and more accurately evaluate the “consumption” of hybrid- and electric vehicles as well. It will likely also include actual on-road testing (most likely in response to the loopholes exposed by Volkswagen's Dieselgate drama). But until the new system is in place, we'll just have to contend with often irrelevant data from manufacturers.