CR170 vRS fuel consumption conundrum

[Brimma]

It's a forum, all I have read is helpful and interesting comments.

If one or more of the posts don't meet your exacting requirements then ignore them.

I'm not quite sure why you find my requirements '"exacting"

Comments which don't tackle the problem, and which merely seem to be 'gloating' about mpg figures, are not in the slightest bit helpful

And neither are yours, but you're welcome to your opinion, as it is a forum

[lol-lol]

About to embark on a 360 mile round trip to Carlisle and back today and the fuel gauge was showing nearly empty

Went to the local station and pur in £69 of diesel @ 137.9p/l, so approx 11 gallons

Outward journey, at reasonably quick motorway speeds, maxidot showed 45.4mpg, homeward journey it showed 44.8mpg

Problem - when I got back tank was showing as almost empty, with a range of less than 20 miles, ie lower than when I filled up

Total milwage recorded there and back was about 368, so that equates to a horrifying 33.45mpg

Now I'm fully aware that maxidot isn't accurate, but surely not 11-12mpg out

Calibration of maxidot speed/distance reading must be about right, as the distance travelled is correct

So what's the crack?

Car's only just been serviced and MoT'd, so I can't think there's a fuel leak

Things I have noticed when looking at fuel consumption and have not been highlighted so far as I can see.

1. Fuel consumption is substantially worse at 77 (geniune) for instance than 70 mph, nearly up to a third as aerodynamic drag is the biggest component of power to hold a cruising speed as it is a cube ratio compared to rolling resistance which is a square ratio.
   
2. Car's fuel computer seems diliberately poor in the mid of the range, in the upper and lower reaches of the tank usage it get get better as if it diliberately overstates fuel usage to suggest you fill up. You may only get ten gallons or so in when it says 20 miles left which would suggest only half a gallon left or so.
   
3. Strong winds on part of the journey can really mess with fuel consumption figures, specially VRSs as they have higher Drag Coefficients than the rest of the range.
   
4. Winter diesel fuel less colorific value.
   

Thank God, or public pressure, the 3.6p a litre tax increase is not happening next week!

[stu220]

Brimma,

Mine is a manual CR, so very similar.

My computer is over optimistic by 3-4 MPG.

The cruising speed makes a huge difference to economy.

My daily commute of 45 miles each way along the A299 (dual carriageway) and the M2, during rush hour at about 60-65 MPH returns a true 50 MPG over a tankful (Vpower diesel).

A long run to Holland loaded with family of 5 and estate boot full, travelling at 130 KPH on cruise control it averaged a true 46 MPG.

I think you need to do a tank to tank calculation and get your maxidot checked.

[nickguzzi]

Brimma is saying he is achieving X-mpg. He seems to be vaguely disappointed with the result his instruments indicate, and now wants explanations. However, he is even more vague in his details of driving conditions, speed, rates of acceleration and so on. Obviously he doesn't really want to know, because then our hobby becomes laid bare as the money pit it really is.

Brimma doesn't have a clue as to his fuel consumption, he doesn't know how much fuel he used, which is a prime requisite for doing the calculation. He even made it even more difficult to do a check by chucking some more fuel in later.

If you compare the standard of construction and calibration of aircraft instruments to those provided in road vehicles, you begin to see why the car ones are intended to provide little more than a guide. (note the prices too).

However accurately the ecu can calculate the amount of fuel going through and engine, it only has a rough indication of how much fuel is in the tank - although in this case perhaps more than the driver.

I was under the impression the milage to empty was calculated using the instantaneous fuel consumption, so the last few miles crawling along will really hammer the M2E readout, just like they do when you do a cold start.

Brimma also fails to give us information on his tyres, their inflation levels, wear patterns or actual dynamic rolling radius. All or any of these will alter the revs/mile, ie the distance covered.

So now we have two unknowns, fuel used and distance traveled, unfortunately, the derived product under discussion relies on these two quantities. Yet we are asked to comment ... on what exactly?

If you don't like paying to use you car, use the train. Ahh, no. that won't work, it'll be at least twice as much and twice as long. Use the phone, email - better yet. You want to see something, there's always google images.

Maybe like me, and I would like to think, most on here, you drive for pleasure, then it cost what it costs. Its your hobby. You can ameliorate the expense by slowing down and driving more "carefully" or maybe consider one of those nice oriental micro cars.

Merry xmas everyone.

[Brimma]

Great rant, Nick, you must feel a whole lot lighter now you've got that off your chest

Here is the bottom line, and the main thrust of my query

Lincoln to Carlisle = 180 miles (that's as recorded by my maxidot and Google maps, which as far as I know isn't based on my maxidot)

MPG recorded on maxidot on the outward journey was 45. something

MPG recorded on maxidot on the inward journey was 44. something

When I got home maxidot said I had a range of 20 miles - say what you like about them being pessimistic, but I'm not going to be the one flying blind, ignoring it, and running my fuel tank dry

I went to the petrol station twice when I got back, once to put £30 in, and again to 'top up' (I don't think the extra 2 miles would have a great effect on the overall figure - with the two fills I put in at least £69 in total, if not slightly more, this being the original amount I put in

Query.......why did maxidot say I was running at 45 and 44 mpg, irrelevant of what speed I was travelling, when the true figure was nearer 33?

If I was 'hammering' it so much that it would cane the fuel economy so badly, why was maxidot so far out?

This isn't about me complaining about the MPG, or not wanting to pay to run my car, as you ignorantly suggest

[mr_awol]

Did you come on/off the throttle a lot?

I've found that by 'feathering' the throttle I can get maxidot to show 50+ mpg. I'm not convinced this would be reflected in actual though (but may try it one day)

[Brimma]

Did you come on/off the throttle a lot?

I've found that by 'feathering' the throttle I can get maxidot to show 50+ mpg. I'm not convinced this would be reflected in actual though (but may try it one day)

Not really, the route is A1, A66 and M6, so majority is dual carriageway/motorway

[ACSC]

Brimma, if you want to "trim" the fuel readings using my VCDS you are more than welcome, would need some guidance from someone who knows how to so it mind as I've not played with it yet.

cheers

ACSC

[Brimma]

Brimma, if you want to "trim" the fuel readings using my VCDS you are more than welcome, would need some guidance from someone who knows how to so it mind as I've not played with it yet.

cheers

ACSC

Thanks for the kind offer, but I know someone who is a dab hand at all things VCDS

[nickguzzi]

Merely an observation of the number of imponderables. Just as an example, how do goole calculate their distances? and did you follow the exact same line through every bend as they used, to ensure you covered the exact mileage they say? We accept our trips/oddos are incorrect, so suspicions should be aroused if oddo and google agree.

My ignorance of you is profound. I included the comment about the train etc. as a rhetorical device, as I thought the next paragraph made clear. Perhaps if I had written "one" rather than "you", or not referred to you by name you could have remained calm?

Anyway, my assertion was that the "miles remaining" can vary hugely, depending on your most recent driving style, rather than for the whole journey. The "facts" on which the maxidot bases its presented information is based on not particularly sophisticated metrology.

The maxidot is a cute toy. Occasionally useful to give you some figures to play with when bored. But a diagnostic tool? I don't think so.

[Brimma]

Nick, I may not have followed the exact same lines as Google maps followed, but be realistic here

You nitpick about me not quoting tyre pressures tyre wear, tread patterns etc etc...........are they really going to make 25% difference to the odometer reading?

Google maps says 180, my maxidot reading was 180 - that, to a man of my limited intelligence, would suggest that distancewise, at least, maxidot isn't very far out

If it had read 140 or 230 I would twig there may be a problem

You seem intent on criticising my explanation of the situation, as opposed to coming up with a plausible explanation for a 25% discrepancy in economy, which is huge even by maxidot terms

[nickguzzi]

I'm not criticizing you or any one else, merely pointing out there are lots of factors which if added together could add up to a significant difference.

We know (don't we?) that slinging on the latest suspension/engine part is not going to automatically give an improvement in any parameter except lightening our wallets. The hard part is optimising the new set of relationships we have imposed on our vehicle.

So what then is your explanation?

Mine is that the metrology is inadequate.

[Brimma]

So it's the modifications now which have lead to the maxidot over-reading by 25%?

Silly me for not having worked that out for myself

Maybe the maxidot is confused by the coilovers and big brakes, and has therefore miscalculated the amount of fuel passing through the engine

You like to use a lot of fancy words and flowery terminology, but in the end say absolutely nothing, but I value your time and perseverence

[Fitz323]

Your maxidot thinks your loaded Bryan due to all the gold on your car so it thinks your not really bothered about mpg...... Helpful I know ;-)

[Brimma]

Your maxidot thinks your loaded Bryan due to all the gold on your car so it thinks your not really bothered about mpg...... Helpful I know ;-)

It's as helpful as someone else's input, John, and probably equally as capable of being the answer

[eccleshill]

This thread really is wandering around

The fuel system works like this:

The fuel pump circulates fuel around a "ring main".

The injectors take what they want from this ring main and the rest is returned to the tank

This prevents the fuel pump pushing against a semi-closed system causing the fuel to overheat in the pump..

The fuel computer uses data from the ECU to add up the amount of fuel it has told the injectors to use. It also records distance travelled so it knows volume of fuel and distance travelled since the last reset and uses this to calculate the fuel consumption

It calculates instantaneous consumption by using rate of fuel usage and current speed - mpg=mph/gph.

MIles to go is calculated from the fuel consumption over the preceding 50km (30 miles) and the volume left in the tank (ie the fuel gauge).

Back to Brimma's problem that needs a logical approach to problem solving

In simple terms it breaks down to - Maxidot said 43/45mpg, fuel usage says 33mpg.

How can this be expalined?

A fuel leak in the ring main. Fuel has left the tank and has not been recorded as used by the ECU so fuel data MPG is going to be lower than maxidot. A leak may not be obvious while standing as the flow through the ring main and pressure will drop when idling or stopped. However, at motorway speeds the flow and pressure will be higher making a leak more likely .

Faulty injectors - they are injecting more fuel than the ECU is telling them to.

Inaccurate Maxidot. Brimma, one missing piece of vital information is the normal correlation you get between maxidot and fuel data. Has it always been this bad or has it suddenly changed on this trip?

If it has always been bad then it needs recalibrating.

If it has previously performed to the normal (in my experience) 5% optimistic then something has changed. WHat?

Fuel leak - see above

Modification - have you changed anything? Did they upload an ECU update at the service that has bu66ered up your remap so that not only are you using more fuel but the maxidot calibration is all to ****?

In the time since the Lincoln<>Carlisle trip has the Maxidot continued to over-read by 25%?

[Brimma]

This thread really is wandering around

The fuel system works like this:

The fuel pump circulates fuel around a "ring main".

The injectors take what they want from this ring main and the rest is returned to the tank

This prevents the fuel pump pushing against a semi-closed system causing the fuel to overheat in the pump..

The fuel computer uses data from the ECU to add up the amount of fuel it has told the injectors to use. It also records distance travelled so it knows volume of fuel and distance travelled since the last reset and uses this to calculate the fuel consumption

It calculates instantaneous consumption by using rate of fuel usage and current speed - mpg=mph/gph.

MIles to go is calculated from the fuel consumption over the preceding 50km (30 miles) and the volume left in the tank (ie the fuel gauge).

Back to Brimma's problem that needs a logical approach to problem solving

In simple terms it breaks down to - Maxidot said 43/45mpg, fuel usage says 33mpg.

How can this be expalined?

A fuel leak in the ring main. Fuel has left the tank and has not been recorded as used by the ECU so fuel data MPG is going to be lower than maxidot. A leak may not be obvious while standing as the flow through the ring main and pressure will drop when idling or stopped. However, at motorway speeds the flow and pressure will be higher making a leak more likely .

Faulty injectors - they are injecting more fuel than the ECU is telling them to.

Inaccurate Maxidot. Brimma, one missing piece of vital information is the normal correlation you get between maxidot and fuel data. Has it always been this bad or has it suddenly changed on this trip?

If it has always been bad then it needs recalibrating.

If it has previously performed to the normal (in my experience) 5% optimistic then something has changed. WHat?

Fuel leak - see above

Modification - have you changed anything? Did they upload an ECU update at the service that has bu66ered up your remap so that not only are you using more fuel but the maxidot calibration is all to ****?

In the time since the Lincoln<>Carlisle trip has the Maxidot continued to over-read by 25%?

Finally, someone has provided a response which has the reasoning to back itself up, I am eternally grateful for that

I am currently keeping a very close eye on the next tankful, and will report back in due course

I took the somewhat dirty K&N filter out and replaced it with the standard paper filter, and on a couple runs the maxidot consumption has been just below and just under 50 - I know in real world terms that is likely to be nearer an official 45, which should give a 12 gallon range of 540

[Kiwibacon]

Nickguzzi has it.

If you really want to know your MPG and nail any maxidot reporting issues. Then you've got to run a log over many tank-fills. We don't have anywhere near this information, not even a single tank fill.

[Brimma]

We don't have anywhere near this information, not even a single tank fill.

For the sake of repeating myself, I put £69 in to fill up before I set off, and when I got back I put between £69 and £70 in, and had done around 368 miles

I shall now, obviously, be watching maxidot like a hawk to see if there are any other huge discrepancies between recorded mpg and actual mpg

[Kiwibacon]

Run a fuel log. Single tanks can swing either way.

[Brimma]

Run a fuel log. Single tanks can swing either way.

I'm well aware of that, but the thread isn't about actual mpg, it's about maxidot telling me I'd achieved 45mpg both ways, when actual was nearer 33

[Kiwibacon]

Did you read that through before posting?

[Brimma]

Did you read that through before posting?

Yes

Although I mentioned actual, the thread is about the 12mpg difference in the maxidot reading

[Kiwibacon]

Using the least accurate mpg checking methods possible.

This thread was started on December 16th. Do you not have any other fill-ups and distances to long since then?

[Brimma]

No I don't yet, as the car doesn't get used that often

One of the reasons for using this particular day was to give it a run out