Unexplainable Fuel Consumption

SuperHans

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Hi, I have a c270 cdi which I often drive about 250 miles weekly up and down the m6. The computer readout reads between 52 and 56 mpg average which to me seems very good. However, if I work out fuel consumption the old fashioned way, tank to tank, I'm working out the consumption at being around 30mpg (£40 got me 250 miles recently).

Now I've heard of the computers being inaccurate but this is a joke!

Have you guys ever seen anything like this? Something wrong with my car?
 

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Maybe part of the same scam as VW, cooking the figures?
 

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"£40 got me 250 miles recently " is meaningless.

The number of litres you used is the only information of any use.
 
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SuperHans

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Yes okay apologies. So 250 miles at £40, each litre £1.07 so 37 litres. It works out at just over 30 mpg, which is appalling considering what the computer is saying! Is there any explanation? Is my car breaking?
 
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Naraic

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Yes okay apologies. So 250 miles at £40, each litre £1.07 so 37 litres. It works out at just over 30 mpg, which is appalling considering what the computer is saying! Is there any explanation? Is my car breaking?

I have done more than 50 mpg in my 320cdi...but then it showed 35mpg on the fill to fill.

If you do 100 miles at 50 mpg, you have used 2 gallons.
If you do the next 100 miles at 25mpg you have used another 4 gallons.
In 200 miles you have used 6 gallons = 33.33mpg.

Unless you brim your tank at the beginning of the motorway and immediately get to say 50 mpg indicated and drive that way for a couple of hundred miles and brim the tank again you will not see close to 50mpg on average.

The on board system is usually optimistic anyway...but I would expect to see 45mpg if I had an indicated 50mpg.

Also remember the overall average is obtained from the "after reset" rather than "after start".
 
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A.J.

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I must admit that like you I would have thought the most accurate way is to measure from fill up to fill up. Could it be that your method of calculation is incorrect ?

Forgive me if I am trying to teach my Grandmother to suck eggs but are you calculating from the Miles covered from last fill up ÷ Total litres input x 5.546 = MPG. That is the method I always use ignoring what it says on the dash. :)
 
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To tell you the truth I don't do it by hand. I use the fuel calculator website (first hit on Google). I can't imagine that is wrong! I must just have an exceptionally optimistic fuel computer coupled with a chronically uneconomical 270 cdi... Sad times indeed
 
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How does the car calculate its own MPG? Does it work it out based on what the fuel tank sender indicates has been used? Or something more along the lines of an injector pulse means X amount of fuel, multiplied by the number of pulses etc,, therefore worn injectors could give more inaccurate readings?
 

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Miles covered from last fill up ÷ Total litres input x 5.546 = MPG.

It's 4.546!


OP: Could be the car is leaking fuel?

Is the MPG on the computer for the whole journey? If so, 50+ is pretty good - I can get mine to that figure, but it takes 100 miles of very steady motorway driving.
 

LostKiwi

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Most cars calculate from injection pulses and a known injector size.
I had a SAAB 9-3 (hateful thing) that I swapped to bigger injectors on. Until the ECU was updated with the new injector size it was overly optimistic by about the amount of the injector change (20% in my case).
Was very wife friendly like that mind - they never complain about fuel consumption when the computer says 45mpg or more for a 2 litre petrol!
 

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It's 4.546!


OP: Could be the car is leaking fuel?

Is the MPG on the computer for the whole journey? If so, 50+ is pretty good - I can get mine to that figure, but it takes 100 miles of very steady motorway driving.

Oops !! That would certainly show an improvement :rolleyes:
 
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mej

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I've been recording mpg for 4 years now on Excel based on full to full, the only accurate method. Always ask for a VAT receipt as that will show litreage. Only a minute to update the evening after a fill, and easily done whilst watching the mind numbing box. I must say, though that my S211 220cdi computer is only 5% or so off. The longer you go on the smaller any filling discrepancies become in the overall calc.
 
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SuperHans

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Yes it's for the whole journey. I have considered the leaking fuel hypothesis... No sign of any on the road though. Yes 90% of my driving is at a constant 78mph on the motorway on cruise control.

It's 4.546!


OP: Could be the car is leaking fuel?

Is the MPG on the computer for the whole journey? If so, 50+ is pretty good - I can get mine to that figure, but it takes 100 miles of very steady motorway driving.
 

Rory

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Yes 90% of my driving is at a constant 78mph on the motorway on cruise control.

You said 250 miles up and down the M6 - is that 250 miles per week, 50 miles per day, 25 miles each way?
 

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I'd go along the lines of worn injectors so the car is actually dumping more fuel in than it thinks it is, best to have it put on STAR and have the live values checked to see how badly worn they are.
 
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You said 250 miles up and down the M6 - is that 250 miles per week, 50 miles per day, 25 miles each way?
125 miles up and 125 miles down once a week.
 

malcolm E53 AMG

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125 miles up and 125 miles down once a week.

Your figures are poor then at 30mpg, my 320cdi would be recording a true 40+mpg over 125 miles, unless you are are averaging 90mph lol.

The IL5 cylinder engine is generally very economical so it appears there may be a problem. Worth checking the air filter has been changed and that you don't have a fuel leak however I'm sure you would know about that. If the car is running well probably better to leave well alone and accept the lower mpg as pursuing the issue may cost £££
 
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I'd go along the lines of worn injectors so the car is actually dumping more fuel in than it thinks it is, best to have it put on STAR and have the live values checked to see how badly worn they are.

Jeeesh, sounds expensive, I can't afford to replace the injectors if they're worn. I'll see if I can get on star and update you
 

V6Matty

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they can be refurbed and I think some places do exchange units, Steve at Avantguard Motors recommended a place he uses for this sort of stuff to me a while ago, I'll see if I can find the name of the place.

edit: Just found the name, its DTM in Bristol and the chap you want to speak to is Ron. Mention you were pointed in their direction by Avantguard Motors and they should look after you.
 
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they can be refurbed and I think some places do exchange units, Steve at Avantguard Motors recommended a place he uses for this sort of stuff to me a while ago, I'll see if I can find the name of the place.

edit: Just found the name, its DTM in Bristol and the chap you want to speak to is Ron. Mention you were pointed in their direction by Avantguard Motors and they should look after you.

Well that's promising, if the issue can be resolved by just changing the injector copper seal, the bolt and the clamp it mightn't cost too much. Unfortunately I live in Walsall, West Midlands so I think that a drive to Bristol might be a little too much out of the way haha! Your input is much appreciated though.
 
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