Now that summer is over, I thought I ought to update you on my experiences in using summer, rather than winter, diesel in my Kia Venga. When I was last in touch, (Issue 333) in early April, I reported running through a tankful of Esso diesel at the rate of 51.2mpg registered on the on-board computer. I then refilled with Shell in mid-April, expecting a tankful of summer diesel, but was disappointed to return only 55.2mpg – better than Esso, but only in line with previous tankfuls of Shell winter diesel. Since then though, each tankful (Shell or Esso) has returned very close to 58mpg. An experimental tankful of Tesco diesel (plus a double dose of additive) returned a below-par 56.2mpg. My conclusion, for what it’s worth, is that winter diesel carries a fuel consumption penalty of three to eight mpg, depending on the brand of fuel. Supermarket fuel seems definitely inferior, winter or summer. Esso winter diesel seemed inferior to Shell, though I’ve not observed the same difference in their summer diesel fuels. 

I confess to being a little disappointed with my summer consumption – I had expected to edge a little closer to the 60mpg mark. More subjectively, I remain unconvinced of the merits of universal distribution of winter diesel in the relatively mild UK. It’s obviously an essential for some users, but I think that most of us would survive most winters on summer diesel and never notice the difference. Perhaps filling stations could offer us the option, just as they currently offer a choice between regular and premium diesel, so we could fill up with either winter or summer grade, depending on our particular needs. I also think it’s unjustifiable to be still feeding us winter diesel in mid-April.

Slightly off the topic, I had an illuminating experience after filling up with my current tankful of Shell. My homeward journey from the filling station is about 22 miles over undulating rural A- and B-roads, which I normally complete with a (flattering and unsustainable) figure of 70+ mpg recorded. On this occasion, diesel particulate filter (DPF) regeneration apparently initiated as soon as I left the filling station, and I was dismayed to reach home with only 46mpg showing – a dramatic illustration of the impact DPF regeneration has on fuel consumption. Happily, with half the tank consumed, the figure now stands at 58.1mpg, so I’m hopeful I might yet improve on my summer’s best! 

Bill Lawrie

Thanks for this update Bill. We’ve covered some of the ground previously – the specific response of the Kia engine to the winter/summer diesel grades, and the probable tank-to-tank variations from atmospheric conditions and traffic conditions that cloud the issue. But there’s no doubt that you are identifying variations in fuel economy between different retail fuel sources and the negative fuel economy effects of winter grade diesel. I’m now fairly convinced by an authoritative source that refinery practice does not change at the supposed season change, but that the required specification changes for winter diesel are generally achieved using additives. Like all fuel additives, the levels of anti-waxing and cetane improver addition for winter diesel can run from the bare minimum to meet the specification to more generous levels, and we have no firm information on how these additives affect fuel economy. Higher cetane values improve starting and general combustibility, but that doesn’t necessarily imply any beneficial changes in fuel economy. The past two or three winters have also generated some fuel system blockage problems that are not the same as normal gelling/waxing up, and are as yet not fully understood, but may be associated with the biodiesel content of up to five per cent.

What I think you have identified for Diesel Car readers is that the variations exist, but they may be very specific to engine manufacturer and local fuel supply in a specific geographical location. What interested or concerned readers should do is go out and test, as you have done, and purchase the fuel that they find runs best in their car – which might not even be the same brand (with or without any after-market additive they may purchase) in summer and winter; unfortunately though, it may well not be the same fuel when they buy that brand elsewhere in the UK! It’s down to the cross-brand system where distribution terminals may supply a number of fuel brands whose quality differs only in the additive packages which they are given, not the properties of the base branded fuel, which may vary significantly around the UK, depending on which refinery supplies the distribution terminal. On top of all this, significant volumes of refined diesel fuel are now being imported from abroad.

On the DPF issue, I experience a similar phenomenon when a DPF forced regeneration takes place with my car. The running mpg drops into the 30 to 40mpg range, and it knocks back the running on-board computer mpg figure – reset every fill-up – depending on how far I have travelled on that tankful, but probably only by around one to two mpg over a whole tankful. It happens generally once or twice a tankful (600 miles plus). There’s nothing amiss, now that I know when and how it is all happening! Thanks again Bill.

Doctor D