Can hybrid electric vehicles beat diesels on mpg?

Hybrids have always had a miles-per-gallon advantage in urban driving but new EQUA Index data shows that they are gaining on diesels in motoway or highway driving and, if current trends persist, hybrid electric vehicles (excluding plug-in hybrid electric vehicles) are set to take the lead in 2017.


The dotted trend lines in the above graph, representing motorway mpg for diesel vehicles and gasoline hybrids tested by Emissions Analytics, are converging. While the downturn in diesel mpg may be due to a change in manufacturers’ focus from fuel economy to NOx emissions, what is more striking is the improvement in gasoline hybrid performance on the motorway as a result of technological advances.

The step change in technology is even more noticeable when European EQUA data is compared to North American EQUA results. The graph below shows gasoline hybrid performance in the US is particularly impressive on our combined cycles. With this level of fuel economy it seems unlikely that diesel vehicles will ever make a significant impact on market share in the US. With the mpg penalty of some NOx aftertreatment systems, perhaps it was to gain a fuel advantage over hybrids that Volkswagen resorted to using a defeat device when bringing their diesel models to the US market.


Another noticeable effect of the different product mix in the US is the level of carbon monoxide emissions. Both regular gasoline cars and gasoline hybrids have much lower CO emissions than their European equivalents, with regular gasolines 30% lower and gasoline hybrids 64% lower. This is despite the fact that the US have a less strict limit, at 2.1g/km, than the EU’s, 1.0g/km limit.

When we last wrote about hybrid vehicles back in October 2014, we concluded they were delivering “good but not best-in-class fuel economy, but [were] typically the cleanest, and if you are a light-footed, congested town driver, they are ideal.” Two years on hybrids, particularly in the US, have really upped their game. They are still a cleaner drive than a diesel and may soon offer better fuel economy wherever you drive them but heavy-footed drivers should still exercise caution.

Do the benefits of stop-start technology outweigh the irritations?

The principle behind stop-start technology makes sense. Turn off your engine whilst it is not needed, for instance at red traffic lights, and you will save fuel and reduce emissions. However, at EA we wanted to know just how much of a difference it really makes and so we mined our database of 900 real on-road tests to find out.

First we looked at the test results from a VW Golf fitted with stop-start technology, crunched the numbers and found it spent 9% of our test idling, or rather in the off-mode. For this fairly typical diesel vehicle, 0.4g of CO2 is saved per second that the engine is switched off. Extrapolate this and, as the table below shows, drivers of this model do see some benefit from having engines equipped with stop-start technology.

During the official New European Drive Cycle (NEDC) the car spends 24% of the time idle. So when we ran our Golf data through this test the resultant fuel efficiency was supposedly even greater.

stop-start technology

Admittedly, the analysis above only considers one vehicle and it is still based on a test cycle, albeit our real on-road cycle rather than in the lab. However, earlier this year for Channel 4’s Dispatches programme, EA fitted a data logger to a real driver’s car and tracked it for a week. Data from this real-world experiment showed that this Seat Alhambra spent 8% of its time idling. Very similar to the 9% of time the Golf was in off-mode but a far cry from the 24% anticipated by the official tests.

Interestingly, our data also shows that exhaust temperature drops by about 40% when the engine switches to the off-mode. When the engine re-starts the temperature quickly rises again. However, this can often coincide with a period of acceleration as the vehicle pulls away, which may mean higher NOx emissions. Thus careful thermal management strategies need to be employed by the manufacturers to ensure their exhaust after treatments are still effective against NOxemissions, which is something EA will be studying in more detail in the future.


Stop-start technology is a particularly attractive proposition for manufacturers looking to achieve a high MPG result and low CO2 rating on the NEDC. Drivers will see some benefits, just not as much as published because the NEDC is flattering the result due to its high proportion of stationary time. So, if you can cope with the annoyance factor of having your engine switch off every time you pull up it is worth having.

Revealed: Annual MPG Report Card

Real-world fuel economy hardly growing

Evidence from our large-scale test programme of passenger cars in Europe reveals the difference between the official fuel economy figures for new cars and their real-world results is continuing to grow.

The latest examination of our data shows that the gap between the combined New European Driving Cycle figures and our real-world results has grown to 24%. This is a dramatic increase from the 16% average variance we first recorded in 2012, and shows the degree to which official figures distort the true picture of vehicle efficiency (dotted green line on graph below).

What this means is that, in real terms, the fuel economy motorists can expect from their new vehicles is hardly growing, just 2mpg over the last three years (blue line), yet the official figures show a marked improvement (red line). While it is true that vehicles went through a phase of significant efficiency improvement in the past, these new results suggest progress in this area has now stalled.


The bigger picture

As older cars are replaced by their owners with newer models, fuel consumption and CO2 outputs should fall.  However,  if drivers switch from larger engines to smaller vehicles this improvement tends to be counteracted. In an earlier newsletter we described how the efficiency gap is greater for smaller-engined vehicles, and particularly downsized engines in larger cars. The very smallest cars are on average 36% below their advertised MPG.

In addition, any backlash against ‘dirty diesels’  may further work against greenhouse gas reductions if consumers switch back to the higher CO2 gasoline vehicles, despite the fact that the industry narrative and public perception about diesels may be lagging reality.  We are seeing that the latest Euro 6 diesels are significantly lower in NOx and particulates as discussed in last month’s newsletter.

The overall effect of these factors is potentially to constrain the UK’s ability to meet its greenhouse gas targets, and to cost the consumer more at the pump.

More on this…

EA has launched a new service for tracking and benchmarking MPG performance. It provides access to our database from high-level trends to detailed results from individual tests. Find out more.

The data we collect is used to populate What Car’s True MPG consumer website. For more information on which vehicles perform best and worst visit  their site.