How reliable are official MPG figures?

Have you ever checked your real MPG with the figures quoted by car manufacturers in their glossy brochures? If you have, then you probably got quite a shock. Real MPG figures can vary quite significantly from the official figures, but why? It’s because the tests used to obtain the figures are not representative of real life driving.

How are the official MPG figures obtained?

The tests are carried out on a ‘rolling’ road in a laboratory, not on a real road (or even a test track!). A sensor is fitted to the exhaust pipe to measure emissions, in particular C02. This figure (calculated in g/km) is used to determine which tax band the car falls in to. The fuel economy figure is derived from the CO2 output, and not from direct measurement of fuel usage.

There are two parts to the test. The first is designed to simulate ‘urban’ driving; the car is started with a cold engine and covers 2.5 miles, with some stopping and starting, averaging 12mph and a top speed of 31mph. The second stage follows straight afterwards while the engine is still warm and is intended to simulate ‘extra urban’ driving on open roads and motorways. The car covers 4.3 miles with a maximum speed of 75mph and an average of 39mph. These tests provide the ‘urban’ and ‘extra urban’ MPG figures published by manufacturers. The weighted average of the two figures provides the ‘combined’ MPG.

It’s not hard to see why it is difficult to get MPG performance in day to day driving as high as the official figures. However there are other aspects to the test which make it even more unlike real life driving. The test is carried at temperatures of 20-20C, this might be realistic if you live in the south of Spain, but not the UK. Cold weather can reduce fuel efficiency quite significantly. There are not many days in the year in the UK when you will drive to work in the morning with temperatures of 25C!

The tests are performed by technicians aiming to get the best MPG performance within the test guidelines. For example, the test allows 50 seconds to accelerate to 60mph. This will put hardly any strain on the engine. Even the most conservative driver in a small engine car would probably do it in 25-30 seconds. Also there are no hills or bends on a rolling road, both of which reduce fuel efficiency.

How accurate are official MPG figures?

The official tests are designed to be low cost, repeatable and ensure that every car is measured in the same way so that comparisons can be made between them. To achieve this most of the variables that would affect real life fuel economy have been taken out of the test, this includes road and weather conditions, the number of passengers, and most importantly driving style. It would not be possible to reflect all these variables in a standard test, however most industry commentators believe that the tests could certainly be changed to more accurately reflect real driving conditions.

The most extensive ‘real life’ fuel economy tests were carried out by What Car? who hired an independent expert to devise and carry out a series of more realistic fuel economy tests. There were 5 parts to these tests: urban, suburban, gentle, brisk and motorway. Tests were carried out on real roads and direct measurement of fuel consumption was made. In all, 26 cars were tested.

Unsurprisingly, the majority of cars achieved a lower fuel economy than their official MPG figures, with only 5 cars (20%) falling within 5% of their published figures. However what is surprising from the tests is the variation in the amount that cars deviated from their official MPG figures. Some cars recorded MPG 20%-30% lower than their official figures, whilst 2 achieved slightly better MPG.

Overall the Super Minis and Small Family Cars showed the greatest disparity from their official MPG figures, being on average 15% lower, whereas the Super cars tested were much closer.

This is because official fuel economy tests include very little simulated motorway driving or brisk open road driving, whereas the What Car? survey included more extensive driving of this type. This type of driving puts a much greater strain on small engine cars compared to more powerful larger engine cars. For example the Toyota iQ 1.0 achieved 46.9 MPG on the motorway compared to the BMW 318d which achieved 53.9MPG. The assumption that smaller engines are always more fuel efficient is therefore not always true, it depends on the type of driving and the driving style.

So how useful are the official MPG figures?

The official MPG tests provide a rough guide to fuel economy, but you should not expect to achieve the published MPG performance on the road. More ‘real life’ mpg data is becoming available from motoring publications and web sites all the time which should provide more realistic estimates, however no MPG figures will give you a totally accurate indication of what fuel economy you will actually achieve in reality due to the large number of variables that can influence performance. If fuel economy is a key buying decision, then look for cars which have generally good MPG performance within car categories that will suit the type of driving you will do. For instance, if you do a lot of motorway driving, then larger engines are more economical in fuel.