Before we get to Petrol versus Diesel, lets ponder for moment on the biggest camp fire stories of motoring: Fuel consumption.

Being outspoken about fuel consumption, I have not only been in the dogbox a couple of times, I sometimes instantly lost friends because of it. Seriously! This is the good stories that you don' t want to spoil with facts. So I had to learn that when people start sharing stories about fuel consumption, it is like talking about the weather. It is mostly plain chitchat to start off the social conversation, but with a twist. The people in the group take the values mentioned as a fact.

Right. Now I will take out my flame thrower and say some hard words about fuel consumption. If you think that you will take this as a personal attack, rather don't read further and skip to the next section.


Why Fuel Consumption Figures are Suspect

Even the most frugal 4x4's have a shocking thirst for those used to sedans. It it therefore essential that one does not have any illusions about what to expect in this regard.

The problem with fuel consumption, is that most people don't have the slightest understanding about how uncontrolled variations and inaccurate recording, influence the answers. With that, people gets both petty and loose about figures depending on their moods. To lend out your ears and accept that information as real, is nothing but gamble. I cannot recall how many times I had to listen to people who claims to comfortably get 8 km/l on 4.0 litre V6 petrol engined 4x4's hammering it out at 140 km/h all the way. But there is always even better and one old chap told me how he got 10 km/1 with his old Toyota Hilux 2.2 4x4 at 160 km/h. And he had to add that he did not think that was too bad for such a vehicle. Good Grief. How does one reason with such idiotic arguments. The person will simply insist on this nonsense!

The website http://www.fuelly.com/ was created in order to give owners the chance to record the fuel pumped into their vehicles, with an ODO reading at each fill up. It should be simple enough selecting the correct vehicle and record litres and an ODO reading? The website then claims to give real world figures for specific vehicles models. In theory yes. But oh my word!

It is nothing but a perfect testimony of GIGO (Garbage In Garbage Out) and proof of how little care people takes when recording data, because they are clueless. Too many could not even manage to register their vehicle under the correct fuel type with the result that average figures calculated for most models, are far from correct since petrol and diesels are mixed. If owners cannot manage to consistently register a diesel under a diesel or a petrol under a petrol, it is not surprising that when one looks at data entered for individual vehicles, there are many instances of some obvious and serious errors in recorded figures. If owners cannot manage to enter this simple data correctly themselves, then it is obvious that they will not be able to, or make the effort to analyse the entries of data to see if the figures calculated, are accurate.

As a professional registered mechanical engineer, I need to be good with taking accurate measurements and record and analyse complex technical data in order to sign off my work. I am legally bound and may stand up in court by what I certify, so I do need to be serious about the process.

Yet, I frequently experience that people will only accept my technical opinion when it suits their believes. When it does not, they will just wipe it from the table and for example quote their accountant friend as an authority on the subject as he should know about money. Or they want to revert to the knowledge of their Grade 12 Science class. Wow!

The reason for this rant is so that you need to understand that to make proper conclusions from fuel consumption figures, takes some real care and a bit of expertise. Expertise in which you can easily school yourself never the less. Look, I know it sounds pathetically simple to write down two figures and do the simple calculations required to find an answer, but in the real world the majority of people is not really concerned about keeping records diligently, let alone to be aware of the conditions on the road that had an influence on the figures.

As a result, they are most of the time happy with ballpark figures so they don't do their calculations on the dot and the result is that the figures they get are very subjective – figures which they don't write down anyway. Then something happens and they get the suspicion that something is wrong. They start for some time to diligently and fairly accurately record fuel consumption and write it down, but they compare it with the subjective figures from the past, that they stored in memory. Since they are not in a habit of recording and writing down, they are not used to the fact that there can be a considerable difference between one fill up and the next in seemingly the same type of circumstances. So when the next figure is suddenly higher, they are absolutely convinced that something is wrong and they take action.

Guidelines for Recording Fuel Consumption

By now I think I made my point and it is time to lay down the ground rules of recording fuel consumption:

  1. First of all, if you want to get scientific about fuel consumption, you cannot use the km/l unit since it is not specific consumption. The best unit to use is l/100km and it is important to understand why. Lets take a consumption of 12 l/100 km that is the same as 8.33 km/l. If the consumption increase to 15 l/100km or 6.67 km/l, what is the percentage difference in fuel consumption? Is it 25% or 20%? Right, now consumption increase from 10 l/100km (10km/l) to 20 l/100km (5 km/l). What is the percentage difference? Is it 100% or 50%? These two examples should clearly indicate why using km/l as a unit for consumption, is not at all correct since the % difference is related to distance and not to fuel consumed. Simpler as this it cannot get.

  2. Do not expect consumption figures to be within 0.1 l/100 km of each other.

  3. Accept that the fuel consumption readout from the onboard computer, is not accurate enough. On some of my cars, they were often at least 5-8% optimistic. Modern, German cars.

  4. The simplest most accurate device that you can buy today for little money, is a GPS. This device is a huge help to check the accuracy of the odometer, and to record real moving average speed.

  5. Using the GPS to check the accuracy of your odometer, is important. Once again, lets demonstrate it with a calculation. 10 litres of fuel used per 100 km is 10 l/100km. 10 litres of fuel used over 105 km, is 9.5 l/100km. However, since this inaccuracy is constant, you can choose not to make the correction, but do understand that most others also don't make the correction and that no one knows whether his odo is 2% or 10% out unless he verified that.

  6. Use the GPS to calculate your real moving average speed. This is important because traffic density have a significant effect on overall average speed. In the city, a lower average speed often results in higher consumption because you spend more time idling. On the open road, a higher average speed almost without exception, results in higher consumption. Even though you may try and keep the needle between 120 and 130 km/h for example or even drive on cruise control, traffic slowing you down inevitably lowers the overall average speeds and when this is diligently recorded, the affect can quickly be seen as a major contributing factor.

  7. When you travel a mix between city and highways, do expect that figures can vary quite a bit more than expected, simply because it is very difficult to accurately record the distance spend on each. With a petrol 4x4 that have a fairly big difference in consumption between city and highway, it is obvious that if the exact percentage of each travelled is not known, one cannot be too picky about the resulting difference. With a diesel, the difference between city and highway is often little and there one can bargain on more predictable figures.

  8. If you are of the type who generally do not fill the car up to the neck at each fill, which is a legitimate reason as some cars do have a problem when the fuel will push out at the first parking in the sun, do accept another variance. The level where the pump stops the first time, is very dependant on ambient temperature, and even more on the exact level of the parking. A parking that is not perfectly level have a significant difference in how the air escapes. A small variance of only 2 litres on say an 80 litres refill, is already a 0.3 l/100 km variance on a diesel 4x4 that returns around 12 l/100 km. What I do, is to fill my cars up to the neck each time. That does give the best results.

  9. When you are on the open road, do understand that wind have a significant effect on consumption. If for example you have a 20 km/h wind right from the front, it consumes as much power from the motor as if you were driving 20 km/h faster in no wind. Since it is impossible for anyone to continuously record wind direction and wind speed throughout the trip to get the combined effect there of, all one can say is that there was wind. And lets be realistic, wind is almost always a factor.

  10. The same goes for level roads and hilly areas. The moment that the road goes up and down all the time, a car works much harder and fuel consumption increase.

The above points illustrate just how many things there are that makes a 5% or a 10% difference in fuel consumption and in the real world it easily happens that a number of 5% factors works together on that day and you can have a 20% increase in consumption. With the above factors, is very possible to apply it in such a way to fool yourself or others less technical, to substantiate what you want to believe.

It is a good idea to record fuel consumption as it is a reasonably good indicator of things that may start going wrong, but with all the variables that has an influence, it takes careful analysis and a bunch of experience to fill in the gaps and make sense of the calculated figures.

Ballpark Figures

Here are my ballpark figures for fuel consumption: Test them and you will see how close it is.

For petrol 4x4 vehicles, almost irrespective of engine capacity, one can expect between 20-25 l/100 km in city conditions. On the highway, you might be lucky to make 16 l/100km but often it is worse. With heavy towing, you will quickly see 20-30 l/100km.

For a diesel, 10-12 l/100km in city conditions are ordinary. On long distance, it can go as low as 9 l/100km if you take it real easy, but if you keep up with the traffic, you will see over 11 l/100 km often. Heavy towing and that diesel will hit 20 l/100km pretty easy, but rarely more.

Be aware that a smaller engine capacity 4x4 will be as heavy a larger capacity 4x4 under normal conditions. When it needs to work hard like with towing, the larger capacity will be even heavier on fuel, but it will make up a bit more speed although probably not enough speed to make it worth the while.

The long and the short is that it takes a bit of time and effort to understand the subject of fuel consumption on a level where it is of real use to anyone. It is not simply two figures that one records and make the calculations.

Shocking? Get over it. You will burn the fuel in relation to the power available of the vehicle and how much of it you use. Forget about all those miracle devices. There is no magic about it.