Doctor Diesel

Disappointed of Diss…

I have just changed my 2013 BMW 520d, which was very economical, delivering 50mpg going to work, and 60mpg on a run, for a new 520d Luxury, which is turning out to be 10 to 15 mpg more thirsty. The ride on the new car is also very hard, presumably due to the combination of 18-inch wheels and run-flat tyres. The dealer says that both fuel economy and ride are normal (How surprising! Doc), and that I cannot compare fuel economy with the old car, as they are different engines. I have read one or two forums, and they all seem to say that the newer cars are much more thirsty. They all seem to think it is down to the emissions control equipment on the newer engine, and that the exhaust is also louder, due to the EGR almost permanently regenerating, although this sounds rather strange. Have you heard anything, and are other makes and models similar? 

My dealer now says they may be able to retune my car for economy. It’s the first time I’ve heard of a manufacturer doing this. My previous car had Prestivo budget tyres fitted (at £75 each) which were very smooth and quiet, and still had plenty of tread left after 25,000 miles, when I sold the car. I will probably replace the run-flats with proper tyres, but I don’t want the expense of buying 17-inch wheels. Overall, I spent a lot of money replacing a fantastic car with a poor one and, to rub salt in the wounds, I sold my old car to someone at work, so I have to look at it every day! 

S Tarmey

I’m afraid that I have to say that running a 5 Series BMW on budget tyres is like going on a luxury Cunard cruise wearing clothes from Primark. But then probably plenty of people do exactly that. Whilst I do sympathise with you, you essentially bought your new car “on paper”, and evidently without a suitable test drive, otherwise you would have noticed the shortcomings of the wheels, tyres, and ride comfort. I do often write about such things in my column.

But the fuel economy is indeed disappointing, and it’s the sort of thing that makes you feel that EC mpg testing has been more and more unrealistic as the years have passed. It seems quite likely to me that, unless you bought your previous car very late in 2013, it will have been a Euro-5 emissions engine, whilst the engine in your new one is Euro-6 compliant. The changes involved tightening up NOx emissions and the fuel economy figures were little changed, on paper, but maybe significant in reality. I have no knowledge though of any problems with particulate filter regeneration on these later previous model BMW 5-Series diesels though.

You have to remember also that the 18-inch run-flat tyres (ugh!) may have quite a high rolling resistance, which could erode the fuel economy by as much as four to five mpg! They are pretty awful things these run-flats fitted to big wheels (that’s my personal opinion!) and BMW in the UK lost business over recent years in the UK because many BMWs did not run as smoothly and comfortably on run-flat tyres as they used to on the conventional tyres that are standard equipment in many parts of the world. Now they have retracted a little, and you can have conventional tyres, with certain smaller diameter wheel designs. The answer trotted out in the UK is to specify the adjustable damping control option, and spend £985 overcoming the shortcomings of the big 18-inch wheels and run-flat tyres, or specify the standard 17-inch alloys with conventional tyres.

I don’t imagine that your BMW dealer is offering an official BMW economy tune ñ I have certainly never heard of such a thing. Find out more about it. But don’t do anything about “tuning for economy” until you have at least 5,000 miles on the clock. Do a cash flow exercise then, using a realistic reduction of 15 per cent in fuel consumption, and you may find that you won’t see your money back for some years. Tuning for economy certainly does work though, and respected tuning company Celtic Tuning do such a conversion, at a very reasonable inclusive cost of £325. You’ll see that their economy conversion ups the power from 181bhp to 213bhp (quite modest) and the torque from 280lb ft to 316lb ft, compared with 238bhp and 326lb ft for their performance tune. So the emphasis is on better torque, and less on power, with the aim of greater engine flexibility, and the ability to use higher gears in any given situation, for better real-world fuel economy.

I will conclude with hopes that you will see some improvement with time and miles in fuel economy though. As for ride comfort, you might find another 5 Series owner who would swap his boring 17-inch wheels with your sexy 18-inch ones! There are such people around! Best regards,

Doc Diesel

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