Hello, and B / C class real-world fuel consumption questions

[Digsy]

Hi all. This is my first post and I joined the forum specifically so I could ask this, so hopefully you all can help.

I've been a happy B180D owner for over three years now but one thing has always bugged me about it, and that is the fact that the on board MPG meter is wildly optimistic: i.e. ever since I bought the car (new in 2016), the MPG meter has consistently told me I am getting between 10% and 20% better MPG than my own measurements tell me.

I do about 20,000 miles a year (personal, not business) so real world fuel consumption is a big issue for me and at the time getting 50MPG was a deal breaker for me buying the B-class. Luckily even taking the above into account, I am still getting a real-world 49.8MPG on average.

But why is the meter so incredibly inaccurate? If anything, I would expect MB to have to make the meter underestimate the consumption for risk of legal action, or something. Also, as the car gets older, the deviation seems to be getting bigger. I have attached an image of my plot of "car" MPG (purple line - approx 60MPG) versus my own measurements (blue line - approx 50MPG). For reference, I take my measurements at each refuelling by using the "from reset" numbers for distance driven and average MPG and then work out my own MPG using the quantity of fuel delivered from the pump - not rocket science. I reset the counters at each refuelling.



The blip at December 2017 and subsequent slight drop in MPG happened after a service, which probably included an ECU re flash.

This brings me on to the second part of my question: I intend to upgrade to a C class estate soon - probably a C220D automatic, so my question is: What real world fuel consumption can I reasonably expect from this? Given that the official NEDC combined MPG for my B180 is 68.9 but I actually get 49.8, I reckon that I can expect about 46MPG from the C-class (calculated from official figure of 64.2 using the same % error).

Sorry for the slightly long-winded first post. Would love to hear your thoughts.

 

[LostKiwi]

Hi and welcome.
Can't answer your queries sorry

 

[Blobcat]

Hello and welcome

They are wildly optimistic and there to give the driver the warm fuzzy feeling that they’re getting decent economy. I’ve not found a single manufacturer that has anything other than optimistic trip computers. I use mileage and fill to fill readings for my cars.

If you change to a C Class try and get one with the OM654 2ltr (launched in 2016) engine. It is very good indeed and fuel economy is excellent. You’ll most likely exceed what you achieved with your B Class.

(If it’s got the 9G box it will have an OM654 engine)

 

[Digsy]

Hi Blobcat & thanks for the fast reply.

Not unexpected, but a bit surprised that an error of that magnitude is allowed. After all, its not difficult to verify, and could legitimately be claimed to be fraudulent if a potential buyer (such as me) uses the MPG meter numbers as a litmus test during the test drive. I guess they have it covered in the small print.

Thanks for the pointers about the C-class engine. I am aiming for a late 2018 model, so I guess that engine would be fitted? I shall look out for it anyway.

Regards,

 

[Blobcat]

Hi Blobcat & thanks for the fast reply.

Not unexpected, but a bit surprised that an error of that magnitude is allowed. After all, its not difficult to verify, and could legitimately be claimed to be fraudulent if a potential buyer (such as me) uses the MPG meter numbers as a litmus test during the test drive. I guess they have it covered in the small print.

Thanks for the pointers about the C-class engine. I am aiming for a late 2018 model, so I guess that engine would be fitted? I shall look out for it anyway.

Regards,

Yes it will have that engine, I had it in a W213 E220d and it was very very good.

 

[BMF]

Hi Digsy,
Welcome to the forum.
I have C220 (W205), it’s on a 15 plate. It has the 2.1 litre Diesel engine and 7g+ autobox. It is a Sport with Premium+ trim level.

Like you, I monitor my fuel consumption by noting mileage covered between fuel stops and doing the math, by my calculations I’m getting on average 47 mpg, I don’t cover the mileage you do but as a guide it’s not far off you estimate. With the car set to ‘Comfort’ and on a motorway run it was showing 70’ ish mpg. The car has the larger fuel tank capacity of 66 litres and I usually fill up fortnightly with around 55 litres.

The thing is, I feel that with the performance and the comfort level of the car I’m a very happy boy, HTH

 

[umblecumbuz]

I think you'll find that many car onboard fuel consumption readouts are very optimistic.
I have noticed this with every car I've had over the last fifteen years.
A manufacturer is not going to deliberately calibrate the system to be pessimistic, is he?

The vagaries of driving conditions and styles are not all calculated with pinpoint accuracy, even in this age where we tend to look at technology as a god - so as the manufacturer has a choice, he will obviously err on the optimistic side.
Good psychology and good for business.

After all, how many motorists go to the lengths that you go to in order to calibrate the exact consumption after every fill-up? Most of us can't be bothered. You are a worthy exception, I think.

 

[Digsy]

I have C220 (W205), it’s on a 15 plate. It has the 2.1 litre Diesel engine and 7g+ autobox. It is a Sport with Premium+ trim level.

Like you, I monitor my fuel consumption by noting mileage covered between fuel stops and doing the math, by my calculations I’m getting on average 47 mpg,

That's great - sounds like I'm not a million miles off then. Thanks for the information

I think you'll find that many car onboard fuel consumption readouts are very optimistic.
I have noticed this with every car I've had over the last fifteen years.
A manufacturer is not going to deliberately calibrate the system to be pessimistic, is he?

The vagaries of driving conditions and styles are not all calculated with pinpoint accuracy, even in this age where we tend to look at technology as a god - so as the manufacturer has a choice, he will obviously err on the optimistic side.
Good psychology and good for business.

After all, how many motorists go to the lengths that you go to in order to calibrate the exact consumption after every fill-up? Most of us can't be bothered. You are a worthy exception, I think.

Well, I view it like any other meter, and I wouldn't expect any other meter to be calibrated to tell porkies just to make the customer "feel" better. There's already enough controversy going on about how relevant the official MPG figures are to real-world driving. Maybe its a deliberate attempt to cover that up to some extent, so when you take the car on a short test drive you don't see a number which is drastically below that which the salesperson just told you?

I know quite a few people who at least periodically check their MPG (I hang out with a lot of nerdy people ) I would have thought this was something much more likely to happen immediately after purchase - my dad certainly did back in the day. But maybe you're right - these days people just trust the meter instead, since pretty much all cars seem to have them included as standard.

 

[umblecumbuz]

There was also a period of extreme cold weather in the last few days of Nomber/first half of December 2017, which may have had an influence on your sudden snap increase in consumption at that time.

 

[Digsy]

Quick postscript to this: I just had a new Volvo V60 on an extended test drive over the weekend so I had the opportunity to calibrate its MPG meter the same way I do mine. Over 230 miles of typical (for me) driving, the car reckoned it was doing 49mpg and when I filled up the maths said it was actually doing 48. So it is at least possible to get a reasonably accurate reading. Interestingly this is also significantly better than the EQUA index of real-world MPG says the Volvo should be (44MPG), whereas for my B-Class, EQUA is a lot closer to my own measured numbers (52MPG).

Its a minefield! Especially if real-world MPG is a selling point and you don't have the luxury of taking every car you consider out on a very long test drive.

 

[rf065]

If you change to a C Class try and get one with the OM654 2ltr (launched in 2016) engine. It is very good indeed and fuel economy is excellent. You’ll most likely exceed what you achieved with your B Class.

(If it’s got the 9G box it will have an OM654 engine)

Hmm, my 2016 C Class has the 9G box but the OM651 engine, how did that happen then?

 

[Blobcat]

Hmm, my 2016 C Class has the 9G box but the OM651 engine, how did that happen then?

Didn’t know they nailed the 9G to the OM651 in the C. I know it came in with the OM654 in the E

 

[rf065]

Didn’t know they nailed the 9G to the OM651 in the C. I know it came in with the OM654 in the E

Mine has it, & my SLC300 also. But as a nod to the OP, I had a brand new C220CDi as a courtesy car and on the m/way in rush hour traffic, it was showing 70mpg without even trying!

 

[Digsy]

Finally time to bring this enthralling story to a close

I took a C220D AMG line estate out for a long drive yesterday and put 130 miles on it doing my regular commute. The MPG meter was showing the same kind of figures that my B class reports but in this case when I filled up and did the maths, it turns out the MPG meter was almost spot on. 57.6 according to the car versus 55.3 actual.

...so I bought one

OK, so that wasn't the only reason why I bought it, but it made a massive difference to the running costs which helped swing my decision a lot.