Can you rely on your car’s mpg claims?

Malcolm, I think it is a misconception that full throttle wastes fuel. It is probably an over-simplification stated to discourage drivers from doing stupid things like accelerating towards red lights. From the Wikipedia page on Engine Efficiency: “Under part throttle conditions (i.e. when the throttle is less than fully open), the effective compression ratio is less than when the engine is operating at full throttle, due to the simple fact that the incoming fuel-air mixture is being restricted and cannot fill the chamber to full atmospheric pressure. The engine efficiency is less than when the engine is operating at full throttle.”

Clint – it all depends on whether you want maximum efficiency from your car or maximum economy. Using your theory, you could travel all day in 3rd gear at say 60mph to achieve “maximum efficiency”, but you would use much more fuel than doing the same trip at the same speed in top gear (say 5th or 6th).

I think most of us would choose top gear to improve the economy – this is after all the main objective of this subject.

AQ – Efficiency translates directly as economy, as it is defined as energy output as a percentage of energy input. (In this case, output energy is mechanical and input energy is chemical.) But this assumes that everything else is unchanged, so your example doesn’t hold. By saying everything else is unchnanged, I mean that under identical conditions (atmospheric pressure and temperature), and identical engine revs, an engine at half throttle is less efficient than an engine at full throttle. The engine at half throttle will be using a large proportion of its petrol just to keep running (the compressions take away a lot of power) whereas the engine at full throttle will be outputting something useful: acceleration. That’s the theory. In practical terms, it means that economical driving entails accelerating at full throttle to reach your desired final speed in the shortest possible time, and then reducing the throttle to maintain that speed for the rest of your journey in the highest possible gear. (I don’t think we are disagreeing, just interpreting the theory differently. And of course, most people would prioritise safety and smoothness before economy.)

Clint – whilst I am fully conversant with the laws of physics in relation to energy input, output and efficiency, you have drawn a direct correlation to economy with your argument – and all this is theoretical and bears no relation to real life driving, which was the original basis for this debate. Even the manufacturers’ MPG figures cannot be relied upon as to what is genuinely achieved on the road. If you believe you will get the best economy by flooring the throttle in each gear until you reach the speed you want, before easing back, you’re on your own – nobody else subscribes to such a practice for maximising fuel economy and nor would they recommend it. Safety and smoothness should, indeed, be high on everyone’s priorities – and are more likely to be achieved when driving with economy in mind, but if you insist that full throttle acceleration to the desired speed is best – then safety and smoothness, as well as economy have gone out of the window.

I brim the tank regularly as I don’t get many chances to fill up in the week. Mine is about 3mpg optomistic. I find the instant read out distracts me from the job of driving and it doesn’t really tell me much in the end. When the car goes into its ‘de-clog filter’ cycle the mpg drops for a while and then creeps back up. Its never below 40mpg and never above 45mpg. While on the subject, I find litres per kilometre meaningless since I never use it. If we converted I’d have to make the effort. I expect it would register in time.

I must go back to working out my fuel consumption, as I did in the days before fuel consumption indicators.. How often does a DPF filter go through a cleaning cycle? My car does indicate when this is being done.

Well, we all know that we are very unlikely to achieve the economy figures quoted by the car manufacturers, but it does not necessarily follow that if a certain model has a better economy figure than a particular rival, that drivers will also find that the same car still beats its rival in real life. Whilst a car promising 70mpg is very likely going to achieve better real life results than one promising 40mpg, quite frequently another rival promising 60mpg turns out to be more economical than the 70mpg car in real life driving. Although car manufacturers have made significant advances in improving fuel economy, the government prescribed tests result in a grave distortion between the publicised figures and what are really achieved. The figures might be as close as 10% worse on some models, but could be 30-40% worse on others. Interestingly, the greater amounts of difference are usually found on those models that promise the greatest economy, whilst conversely, the claimed fuel consumption of performance cars are usually much closer to what is achieved in real life – and sometimes even exceeded. So the differences probably come down to the manufacturers’ priorities model by model – where the primary aim is fuel economy, you must expect real consumption to be rather worse than the claim. Where fuel economy is lower priority for a particular car, then your own results are likely to be very much closer. As with anything that any government meddles with (of any persuasion) take the figures with a very large pinch of salt – best to rely on real figures.

“the claimed fuel consumption of performance cars are usually much closer to what is achieved in real life” – I’m a bit surprised by that. I would have thought that someone who bought a performance car would be more likely to drive it enthusiastically, with less consideration being given to fuel economy.

I don’t believe that these tests involve cold starts, though Which? does include a cold start in one of their tests. Cars are much less economical before the engine reaches the correct operating temperature, so it is hardly surprising that many of us do not achieve the fuel economy figures published for our vehicles.

There are big differences between the speed we drive and traffic congestion we experience when driving, hence it’s unlikely that we will ever have official figures that we can rely on, but I agree that we deserve more useful figures.

I predicted that car manufacturers would try and delay the introduction of the WLTP tests and was not surprised to read articles like this one last year: https://www.transportenvironment.org/news/carmakers-pushing-3-year-delay-tougher-co2-tests

Whatever happens, we still need tests that are carried out under standard conditions for all vehicles – and no cheating by over-inflating the tyres and taping up the doors please.

The EC standardised tests – whether the outdated NEDC or the new WLTP – are designed to give you comparative mpgs derived under standard repeatable conditions to give a comparison between vehicles. That is what they state and manufacturers currently have to publish only these official figures (I believe). The newer test is designed to take account of modern car technology and current driving conditions so should be closer to what many people achieve in practice. We all, however, drive differently – how we accelerate and brake, the terrain we are normally in, how loaded the car is, whether we drive mostly town, rural, fast, slow etc etc. The best we can do in addition to standard tests is to accumulate mpgs from as many motorists as possible and present that – either as an average or some other representation. You can find this information on a number of websites, such as Honest John. I’d suggest using that data together with the official data to make an assessment.