Brochure Mileage vs Real World Mpg

Someone is lying

When buying a car, one of the bigger points of interest is fuel economy. Unless you found a way to get fuel at the cost of an ice-cream, then you are most likely to be concerned with how much your new car is going to consume on the road.

There’s an interesting issue rising at this point. For those who have been keeping an eye on the car market and had the chance to drive more than their personal vehicle, it’s common knowledge that the mileage boasted on the car brochure doesn’t usually match real world testing.

So, why does it happen? Is it just marketing? Not quite. Let’s have a look at a few elements that influence advertised fuel economy numbers and why automakers aren’t (completely) at fault.

Mileage testing

First of all, let’s have a look at how automakers reach the fuel economy numbers they so happily show in the brochure. The key word here is “loss”.

Normally, the fuel consumed by an engine is directly proportional with the energy produced over a period of time. Two issues rise here:

  • The efficiency of the engine is different on various running conditions
  • Various real-life conditions will reduce efficiency even more and increase fuel consumption

Although it may differ slightly depending on testing standards, automakers generally compute mileage when the engine is running at peak efficiency. Please keep in mind that peak efficiency is different from peak performance. At high efficiency, the engine consumes the least amount of fuel to produce power, while at peak performance the engine pushes out as much power as possible without regard to fuel consumption.

Next up, are the things happening in real world. To start off, let’s imagine a car driving on a random road in your city. Aspects like the road surface quality, wind, outside temperature, road incline and altitude, can all affect the final fuel numbers. On the inside, things such as luggage, weight of the passengers, air conditioning, jack-rabbit starts, heavy braking, driving at high (or low) rpm, will also place a great toll on the amount of fuel consumed by the engine each second.

This being said, it’s truly impossible for automakers to provide spot-on predictions on fuel mileage. Still, they still have some explaining to do when the numbers are well out of bounds.

Testing standards

The states in European Union require a testing standard involving two driving cycles. The first cycle simulates a 4km drive at 18.7 km/h to account for urban traffic conditions. The second cycle simulates a 400 second run at an average speed of 62.6 km/h to account for highway driving.

Obviously, such tests do not take in account all the outside factors such as weather or road conditions. However, numbers tend to be closer to real world testing than most automakers data.


Am I getting fooled by the brochure?

The answer is yes… and no. While automakers will tend to maximize fuel efficiency numbers for the sake of marketing, it is also impossible to perfectly pinpoint how much a car will rate on a particular scenario.