Do we still need to mind the MPG gap?

The gap between official miles per gallon and real world mpg has grown to 22%. This is up 5% since we first started testing fuel economy almost three years ago.

The average official combined miles per gallon of the 459 passenger cars we have tested is 57 and this is increasing by approximately 1.7 mpg per year. Real world miles per gallon (TMPG) on the other hand, which averages 44 mpg, remains flat thus causing an increase in the gap of about two percentage points each year as can be seen in the graph below.

NewJune 14 linear v2 Picture

MPG vs. Engine size

The graph below shows that broadly speaking the gap grows as the engine size reduces. If you buy a five litre car you will not get great mpg but at least it will be consistent with the salesman’s patter and most likely your expectations. However, if you are shopping for a frugal run-around you are better off looking at the one to three litre engines which give the best absolute performance as well as a lower divergence from official figures than the super minis.

June 14 v2

Fuel economy by engine type

Our data also shows that petrol engines, as expected, have worse fuel economy than diesels but interestingly the gap to official is also larger. And, manuals return a better fuel economy than automatics but automatics have a smaller gap between official and real-world figures.

June 14 chart 1b

June 14 chart 2b

More MPG

We’ll be looking at MPG in more detail in next month’s newsletter, including an analysis of the manufacturer leader board. The published results will be anonymised but OEMs are welcome to email me if they would like to find out how they sit within the table.

* The original Transport & Environment report Mind The Gap can be found here

Do eco tyres really save you money?

Preliminary tests have shown that ratings on tyre labels are not telling the full story. At mid-range speeds, an F-rated tyre performs as well as a B-rated tyre for fuel economy.

We tested two contrasting sets of 175/70 R14 tyres on the road. One set was a standard tyre with B-rating for fuel economy and the other had an F-rating. The test route incorporated a range of steady-state speeds from 40mph to 70mph on tarmac in consistent ambient temperatures.

May 14 chart

You can see from this graph that the B rated tyre was superior in the 40-70 mph range by an average of 3.8% mpg and 3.4% less CO2. There isn’t much in it at the mid-range speed but a performance gap opens up at 55mph and by the time you get to 70mph the fuel economy has improved by 12.9%.

Thus a consumer buying B rated tyres is unlikely to notice a fuel economy benefit if the journeys they customarily make are mainly urban. Whereas a consumer heading up and down the motorway each day should enjoy an improvement.

Now this was an unashamedly quick and dirty investigation but it does demonstrate that the relationship between rolling resistance and fuel economy is not linear and that to bring real improvements to the way tyres are bought and sold manufacturers need to adopt more sophisticated models.

The current tyre labelling system, made mandatory by the EU in November 2012, is not working. In a report compiled by the National Tyre Distributors Association (NTDA) and LANXESS, the manufacturers of high-tech rubber for tyres, it was found that one year on 93% of tyre retailers said customers never or only occasionally requested information on the label and only 30% knew that tyres affect fuel consumption.

We think manufacturers need better models to translate rolling resistance calculations into fuel economy effects. Improved, independently verified testing and labelling, perhaps with a monetary quantification of the typical benefit would provide a tangible benefit that the consumer would welcome.