Probably asked, answered MPG

[Zolly]

My previous MB was a W204 C250 Diesel and on mixed roads it averaged @ 40-45mpg

However after looking around various car sites and comparing the average true mileage to the current car I have a C200 AMG Petrol auto on the same runs is way below that off @38mpg ( mine is 33) My question is what could be the cause is such poor figures?

I appreciate that Petrol doesn’t pack as much punch re the force of the explosion as diesel. I have no electronic faults reporting off my link . I did recently have a new Nox sensor fitted, as the old one failed. But other than that I am stumped as to why the return is so low. I haven’t driven a petrol for over 20 yrs but now my mileage doesn’t justify the cost I am beginning to wonder if the switch to Petrol was worth it ?
I used to do 25K per year and when on the motorway was getting 60-65mpg easily in the diesel) and clocked over 165K in it before it had to many little things wrong with it that with MB parts would have made it uneconomical to carry on

Has anyone else been had the displeasure of seeing your switch to what you thought would be the more economical way turn to dust. I have no complaints about the car, just the absolute pants of the mpg return

 

[LostKiwi]

Petrol will never be as economical as diesel simply because the calorific value of the fuel is lower coupled to lower overall efficiency. Why were you expecting it to be better?

You are of course producing less toxic NOx and particulates and don't have a DPF to baby round town.

 

[Zolly]

Petrol will never be as economical as diesel simply because the calorific value of the fuel is lower coupled to lower overall efficiency. Why were you expecting it to be better?

You are of course producing less toxic NOx and particulates and don't have a DPF to baby round town.

Thanks for the input . I don’t know why I expected it to be better, I just did. I don’t do as many miles now and from many online calculations when people have posted on forums the result is that you are doing over 15K per year Petrol is the most cost effective. I appreciate no DPF etc .. I have now realised that “ It is what it is” and to lower my expectations.

Thanks

 

[BillyBoy]

I had a C200 convertible as a courtesy car for a few days a couple of years ago. At the time I had an E220d estate and that typically returned 46-48 mpg on my 30 mile commute to work. The C200 returned about 38-39 mpg for the same journey. Never quite broke the 40 mpg barrier. 33 mpg sounds a bit low, but not massively out depending on the traffic conditions. My E400d estate does about 42 mpg on the same run .

 

[John Laidlaw]

The claimed figure is often way off as it relies on tyres pumped up to ridiculous pressures ( not pleasurable driving) and the AC switched off (not pleasurable driving)
Conclusion: to enjoy your driving relax your wallet opening...I think this shall be the new mantra

 

[A.J.]

My previous MB was a W204 C250 Diesel and on mixed roads it averaged @ 40-45mpg

However after looking around various car sites and comparing the average true mileage to the current car I have a C200 AMG Petrol auto on the same runs is way below that off @38mpg ( mine is 33) My question is what could be the cause is such poor figures?

I also have a C200 petrol and on a recent 220 mile round trip it produced an average fuel consumption figure of just over 44 mpg with the Air Conditioning on !. Maybe you shouldn't drive in your diving boots

 

[Blobcat]

The claimed figure is often way off as it relies on tyres pumped up to ridiculous pressures ( not pleasurable driving) and the AC switched off (not pleasurable driving)
Conclusion: to enjoy your driving relax your wallet opening...I think this shall be the new mantra

You need to use yours more...

I suppose it’s one way of improving your fool economy - never actually using any...

(could also do with throwing a bucket of water over it... frankly I’m surprised at you )

 

[Craiglxviii]

The claimed figure is often way off as it relies on tyres pumped up to ridiculous pressures ( not pleasurable driving) and the AC switched off (not pleasurable driving)
Conclusion: to enjoy your driving relax your wallet opening...I think this shall be the new mantra

Correct, actual practice is to run the rolling road test at 26 C (but huge, huge fans to provide surplus cooling air at higher velocity than car equivalent speed) and tyre pressures of well over 50psi.

 

[Craiglxviii]

I also have a C200 petrol and on a recent 220 mile round trip it produced an average fuel consumption figure of just over 44 mpg with the Air Conditioning on !. Maybe you shouldn't drive in your diving boots

My brand new 65 plate E220 CDI never, ever made more than 42mpg on a run, and 33-35 combined.

 

[d215yq]

More modern car = more effort and technology spent legally beating the test so the further away it is from the claimed figure. Hence the more dissapointed the customers are...

 

[d215yq]

Correct, actual practice is to run the rolling road test at 26 C (but huge, huge fans to provide surplus cooling air at higher velocity than car equivalent speed) and tyre pressures of well over 50psi.

Can I ask why they are allowed to do these laboratory tests? Surely there is an approved flat mile stretch of road somewhere that an independent asssessor can drive the car on with the tyre pressures at factory spec when there is less than 5km/h wind in any direction? That would be a way more accurate and fair test than anything on a rolling road. I appreciate "urban" may need something very slightly more complicated, though by no means impossible but for sure the "on a run" figure at constant speeds could be done in this way.

In my W124 handbook there was no legal need for fuel economy test but Mercedes did it themselves in that way - constant 90kph, constant 110kph on a real world road. They obviously erred on the side of caution too as I beat their figures in real world driving by over 20% 33 years and 300k miles later.

The current set-up seems to me the equivalent of awarding NCAP stars based on deformation of the bumper under x kPA of pressure rather than actually seeing what happens to dummies when actual crashes happen. If they can manage fair real world NCAP crash tests I'm sure they can organise some fair real world driving between makes. In fact if they give me the cars I'll do it for them - When I drive to Madrid and back (500miles, 95% empty motorway) which I used to do regularly, tank to tank with the same driving style I would always be within 1-2mpg. And the extra was always in summer so temperatures being equal it was literally 0.5 of an mpg difference when i went at the same speed!

 

[Blobcat]

Can I ask why they are allowed to do these laboratory tests? Surely there is an approved flat mile stretch of road somewhere that an independent asssessor can drive the car on with the tyre pressures at factory spec when there is less than 5km/h wind in any direction? That would be a way more accurate and fair test than anything on a rolling road. I appreciate "urban" may need something very slightly more complicated, though by no means impossible but for sure the "on a run" figure at constant speeds could be done in this way.

In my W124 handbook there was no legal need for fuel economy test but Mercedes did it themselves in that way - constant 90kph, constant 110kph on a real world road. They obviously erred on the side of caution too as I beat their figures in real world driving by over 20% 33 years and 300k miles later.

To make the tests comparable, external factors are eliminated such as wind and rain which could affect the results.

 

[d215yq]

To make the tests comparable, external factors are eliminated such as wind and rain which could affect the results.

Yeah, because if you said wind of less than 5km/h and no rain they could only do the testing 180 days a year...

 

[V6Matty]

Yeah, because if you said wind of less than 5km/h and no rain they could only do the testing 180 days a year...

How about humidity, air temperatures, the person sat behind the wheel in a bad mood, all factors the lab tests remove to get a consistent result

 

[Blobcat]

Yeah, because if you said wind of less than 5km/h and no rain they could only do the testing 180 days a year...

Is that wind from behind or in front? both would alter the figures, any incline allowed? If so I'll take downhill with the wind behind me

 

[John Laidlaw]

You need to use yours more...
View attachment 60690
I suppose it’s one way of improving your fool economy - never actually using any...

(could also do with throwing a bucket of water over it... frankly I’m surprised at you )

least its protected from the seagull poo- genius!

 

[Craiglxviii]

To make the tests comparable, external factors are eliminated such as wind and rain which could affect the results.

Correct.
To make the results of different vehicles (or indeed any other article) comparable, the tests have to be designed in such a way as to minimise or remove variables.

For crash testing, that means that test engineers had to study thousands and thousands of different real life impacts to determine a series of “standard” impact types. Front, rear and side impacts are all simulated using a big block of steel swung into the test car using a hydraulically actuated pendulum. Note, this is tested at various cases of scenario, so the rig can simulate 30mph, 50mph, 70mph etc. They can also precisely align the car under test so performance at different angles of impact can be measured.
Trying to do all of that by driving cars into a wall just wouldn’t produce the level of precision of data needed to make safe design changes. There’s a reason the scientific method is used.

For emissions tests, the car is tested pretty much in the same way. It’s done in an environmentally controlled chamber; the engineers and drivers don’t do anything beyond what you could do at home to improve vehicle fuel efficiency. So, quite truthfully, the difference between claimed mpg and actually- realised mpg is the cost to you for your own personal comfort, in cabin temperature, ride performance, driving style etc.

That the test doesn’t reflect real- world conditions is another matter entirely, and that has nothing to do with the car makers, more with the standards (in this case, please speak to the EU!) applied to them.

The claimed mpg for my 20yo CL500 is 26- 29mpg (sources vary) extra urban. I can regularly beat that, measured fill to fill, just by keeping my tyre pressures checked and driving steadily (but not necessarily slowly).

 

[Craiglxviii]

Yeah, because if you said wind of less than 5km/h and no rain they could only do the testing 180 days a year...

On what road? What is the level of inclination of it? What altitude ASL? What is the coefficient of friction of its surface and how does that change over its length (and over time)?

All these factors affect a car’s performance.

 

[d215yq]

I appreciate all the points. The question would be, if we had a sepc of:

No rain, wind less than 5km/h any direction, incline less than x, constant speed for x seconds, then accelerate to x, etc (this bit could be done by computer as it is in the test but with the human there to take over and abort the test if something happened), tarmac surafce, altitude between 0-400m etc - of which there would be many test tracks/airfields meeting that spec on many days a year.

Then would the results be more or less like the real world results than the lab tests. My bet would be they would be. The trouble is in controlling variables that should be constant the lab test also keeps contant or incorrect those that should not be (e.g. airflow, aerodynamics, drag from tyres at correct pressures).

I would bet my house that the "real world" test would be not perfect but accurate, hence why it is not done, because then industry would have to admit it's not really improving fuel economy and govt couldn't do environmental "scrappage" shcemes, etc.

 

[Craiglxviii]

No. The “real world” tests you suggest would then require:

a) if a test track, way more availability than it has, as environmental emissions test chambers are used pretty much solidly and test tracks ditto.
b) if a disused runway, someone to take over and pay for the surface upkeep and maintenance, then see a) above.
c) if a road, then other traffic will have to be managed off it for the duration of the tests.

The reality is that the test chambers are by a very long way, the best way of actually delivering variables- minimised results in accordance with the test requirements as specified by the supra- national standards agencies. The problem isn’t with tests being done in “lab” conditions by eeeebil carmakers (well, unless you’re VAG). It’s how those tests are proscribed by the authorities that the carmakers must comply with. One plays the game by the letter of the rules (again, unless you’re VAG) and therefore the carmakers are free to explore any avenue within those rules to improve their performance claims.