Book MPG compared to actual MPG

mpgMany drivers have complained that their cars’ fuel economy does not match that stated by the manufacturer. With this in mind, the purpose of this article is to determine how fuel economy is calculated and to compare these differences.

How manufacturers calculate miles per gallon

To test an mpg, each car is placed on a machine called a dynamometer in order to simulate the driving environment in much the same way that exercise bike simulates the real-world cycling experience. Factors such as wind resistance may also be simulated by varying the energy used to move the rollers, which may also be adjusted to account for the specific weight of each car.

When placed on the dynamometer, a professional driver is employed to run the vehicle through a series of standardised driving routines. Again in a similar way to the more advanced exercise bikes. This is to simulate trips through the city, or on motorways, using a computerised display to track his or her progress. To make the whole process a fair test, each routine specifies the exact speed at which each car must travel during every second of the test.

Measuring Fuel Use

In the case of cars with regular fuel (petrol, diesel) as well as natural gas, a hose is connected to the exhaust pipe to collect exhaust gases to calculate the amount of fuel burned during the different trips made by each car.

The Volkswagen Golf TDI BlueMotion

Volkswagen recently claimed that this car is ‘as good as it gets’ in terms of fuel economy when using the internal combustion engine, using an official combined 88.3mpg mpg on average. Whereas real-world road tests carried out by an average real-world economy of only 64.2 mpg. The actual figure is very impressive for a car driven at normal pace as opposed to hypermiling.

Reasons for the differences

Of course, the average used car found will not compare to its brand new equivalent at the likes of, as the former’s components will be more worn and therefore less efficient.

The conditions of factory tests are so closely regulated that it may be argued they are not an accurate reflection of actual fuel efficiency. Conditions in real-world tests are not regulated to the same exacting standards.



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