Differences between Lab and Road Tests
What Volkswagen had been doing was installing software onto its cars’ computers called a ‘defeat device’, which recognised when the cars were being tested and limited their NOx emissions. Then once they were out on the road, the emissions would rapidly increase again. Worldwide, roughly 11 million cars were installed with the software with 1.2 million of them being in the UK.
The MPG figures shown in adverts for cars are obtained through a sequence of tests called the New European Driving Cycle which hasn’t been updated since 1997. Although the tests are performed under lab conditions, electrical features that would otherwise increase fuel consumption due to the power they require remain switched off. Some people argue that as such, the way that cars are laboratory tested is obsolete because certain technologies that have a large impact on fuel consumption such as heated seats, air conditioning and four-wheel drives have evolved significantly since the tests were first introduced. If these things are all switched off during lab tests, then it stands to reason that the fuel consumption levels resultant of different driving styles out on the roads are naturally going to be higher than those recorded in a lab, even before the ‘defeat devices’ are applied.
Other on-the-road factors than can affect a car’s MPG capabilities and are nearly impossible to truly replicate in lab tests include:
- Traffic conditions
- Weather conditions
- Road gradient
- Road type (urban, rural, motorway)
- Vehicle load
- Vehicle maintenance condition
While advancements have legitimately been made in reducing real world emissions, it’s due to discrepancies such as these that the MPG statistic on your car as advertised may be far better than reality.
Developments since the Scandal
Despite occurring over a year ago, Dieselgate is still resonating through the motoring industry today. Many other car manufacturers have been caught out taking part in various fuel economy wrongdoings during testing since; Mitsubishi was revealed to have been artificially boosting its official fuel economy ratings by up to 10% by overinflating tyres. Mitsubishi’s fuel economy deceptions then ended up being revealed to have been going on since 1991, which has affected millions of vehicles. Vauxhall came under fire in May 2016 after their cars were found to harbour software that would turn off emissions controls during on-the-road driving. Chevrolet, GMC and Buick cars in America were found to be sold at advertised fuel economy levels a full 2 MPG better than the EPA’s official numbers. Fiat’s European 500X car was found to shut off its emissions controls almost completely after just 22 minutes of driving. Volkswagen, the originators of the scandal, are still facing various problems with their US CEO resigning in March 2016 as a result of a “mutual agreement” with the company, the ongoing expensive rollout of fixes for their models affected by the scandal and even more expensive buybacks of affected cars from customers with additional compensation pay-outs.
How to Calculate Your Real MPG
At this point, you’re probably wondering how to determine your car’s real MPG. Luckily, the solution is easy. Some cars with trip computers may have an MPG reading displayed on the dashboard, but for the most peace of mind it might be ideal to calculate it yourself. To calculate your car’s real MPG, simply perform the following:
- Fill up your car’s fuel tank until the fuel pump cuts off.
- Reset the trip meter reading on the dashboard so you can then record how many miles you drive until the next fill up is needed.
- Drive normally until you next need to fill up the tank.
- Fill up the tank up completely again and make a note of how many litres of fuel you put in.
- After fuelling, make a note of the reading on the trip meter before resetting it again.
At this point, you should have two numbers: the number of miles since you last filled up and the number of litres you’ve just filled up your tank with. From here, calculating your MPG is simple: just divide your number of miles by the number of litres, then multiply the result by 4.54, like so:
Number of miles / Number of litres x 4.54 = Your car’s real MPG
Example: 350 miles divided by 30 litres = 11.6
11.6 x 4.54 = an MPG of 52.7 (or 53 if you want to round it up)
Alternatively, you can input your number of miles and number of litres into an online calculator to receive your real MPG if you don’t feel like doing the maths yourself.
As an added bonus, Honest John provides an online service that allows you to then compare your car’s MPG to that of over 110,000 other users who have calculated and submitted their car’s MPG to the site. As well as then being able to see how much higher your car’s advertised MPG was compared to its real level, this is an easy way to see if your car’s MPG is typical for that model or if there are issues that need to be addressed.
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